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UNIVERSITY  OF  CALIFORNIA. 

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THE    EARTH'S 
BEGINNING 


BY 

SIR  ROBERT   STAWELL  BALL 

LL.D.,    F.R.S. 

AUTHOR  OF  "THE  STORY  OF  THE  HEAVENS,"  "STAR-LAND,"  ETC. 
LOWNDEAN  PROFESSOR  OF  ASTRONOMY  AND  GEOMETRY 

IN    THE    UNIVERSITY    OF   CAMHRIDGE 


WITH   FOUR   COLORED  PLATES 
AND  NUMEROUS  ILLUSTRATIONS 


NEW    YORK 
D.    APPLETON    AND    COMPANY 

1903 


A r' 


COPYRIGHT,  1902 
BY  D.   APPLETON   AND   COMPANY 


Published  May,  1902 


Copyright  1901  by  Cassell  and  Company,  Limited 


PREFACE 

I  HAD  often  wished  for  an  opportunity  of  attempting 
a  popular  exposition  of  that  splendid  branch  of  Astron- 
omy which  treats  of  the  evolution  of  the  Earth,  the 
planets  and  the  sun  from  the  fire-mist. 

The  opportunity  was  given  me  by  the  kindness  of  the 
managers  of  the  Royal  Institution  of  Great  Britain. 
They  entrusted  to  me  once  again  the  honourable  duty  of 
delivering  the  course  of  Christmas  Lectures  adapted  to 
an  audience  of  young  people. 

The  Lectures  were  accordingly  given  last  winter,  and, 
after  some  omissions  and  some  additions,  they  are  set 
forth  in  the  present  volume. 

I  owe  many  thanks  for  aid  rendered  in  the  illustra- 
tions. The  Eoyal  Society,  The  Eoyal  Astronomical  So- 
ciety, The  Greenwich  Observatory,  The  Lick  Observa- 
tory, The  Yerkes  Observatory,  have  all  contributed;  and 
so  have  my  friends,  Sir  W.  Huggins,  K.C.B.,  Sir  D. 
Gill,  K.C.B.,  Dr.  Isaac  Koberts,  Dr.  W.  E.  Wilson,  Pro- 
fessor J.  P.  O'Reilly,  and  M.  Flammarion. 

I  am  also  deeply  indebted  to  Dr.  A.  A.  Rambaut  and 
Mr.  L.  E.  Steele  for  their  kindness  in  correcting  the 
proofs. 

ROBERT  S.  BALL. 

Cambridge  Observatory,  2nd  August,  1901. 

179898 


CONTENTS 


CHAP.  PAGE 

I. — INTRODUCTION     ........      1 

II. — THE  PROBLEM  STATED 21 

III. — THE  FIRE-MIST  .        .        ....        ..      .        .     39 

IV. — NEBULA — APPARENT  AND  REAL    ....        52 

V. — THE  HEAT  OF  THE  SUN .75 

VI. — How  THE  SUN'S  HEAT  is  MAINTAINED        .        .        95 
VII. — THE  HISTORY  OF  THE  SUN        .        .        .        .        .121 

VIII. — THE  EARTH'S  BEGINNING 122 

IX. — EARTHQUAKES  AND  VOLCANOES        .        •    ;  •        •  158 

X. — SPIRAL  AND  PLANETARY  NEBULJS          .        .        .      191 

XI. — THE  UNERRING  GUIDE       .        .        ,        v       •  •      •  207 

XII. — THE  EVOLUTION  OF  THE  SOLAR  SYSTEM      .        .      246 

XIII. — THE  UNITY  OF  MATERIAL  IN  THE   HEAVENS  AND 

THE  EARTH   .     •  .        .        ...        .        .  261 

XIV. — THE  FIRST  CONCORD      .        .        .        .        .        .      294 

XV. — THE  SECOND  CONCORD       ....        .        .        .  308 

XVI.— THE  THIRD  CONCORD      .        .        .        .        .        .      324 

XVII. — OBJECTIONS  TO  THE  NEBULAR  THEORY    .        .        .  337 
XVIII. — THE  BEGINNING  OF  THE  NEBULA  .        .        .        .      348 

XIX. — CONCLUDING  CHAPTER 361 

APPENDICES    .  369 


LIST  OF  PLATES 

An  English  Sunset  ringed  by  Krakatoa        .  .  •       Frontispiece 

Showing  Localities  of  Earthquakes           .        ,  .To  face  p.  175 

The  Early  Stage  of  the  Eruption  of  Krakatoa  .               "          180 

The  Solar  Spectrum          .         .         .        '.         .  '    .           "          272 


LIST  OF  ILLUSTEATIONS 

PIG.  PAGE 

1.  Iramanuel  Kant   (from  an  old  print)  •         ,         .  .       7 

2.  A  Faint  Diffused  Nebulosity          V  .         .      .  .  .         17 

3.  The  Crab  Nebula         .         .         .         .  .         ...     19 

4.  Jupiter      .         .        ...         .         .  .  .  .         25 

5.  Nebulous  Region  and  Star-cluster       .  .         .-•'«'  .33 

6.  The  Great  Nebula  in  Orion     .         .  ..         .         .  .         41 

7.  The  Dumb-bell  Nebula         .         .        .  .        .      '  .  .45 

8.  The  Crossley  Reflector    .         .         .  .-       .         .  .         49 

9.  The  Cluster  in  Hercules      .         .         .  .         ,         .  .53 

10.  Spectra  of  the  Sun  and  Capella 62 

11.  Spectrum  of  Nebula  in  Orion  and  Spectrum  of  White  Star     64 

12.  Solar   Spectra   with   Bright   Lines   and   Dark   Lines   dur- 

ing Eclipse     .         .         .         .         .  •       •  '•'.'.         .  .69 

13.  The  Nebulae  in  the  Pleiades     .         .         .         .        V        .  71 

14.  The  Sun .       t>;        .  .81 

15.  I.  Spectrum  of  the  Sun.     II.  Spectrum  of  Arcturus        .  85 

16.  Brooks'  Comet  and  Meteor  Trail        .         .         .    .    .  .89 

17.  Argus  and  the  surrounding  Stars  and  Nebulosity   .         .  103 

18.  Trifir  Nebula  in  Sagittarius        .         .         .         .         .  .  105 

19.  To  illustrate  the  History  of  the  Sun       .         .         .       „.  113 

20.  Solar  Corona        .         ,         .         .  ,      .  '      .         .         .  .117 

21.  The  Great  Comet  of  1882        .         .         .                  r  ^ '  .'  119 

22.  Special  Thermometer  for  use  in  Deep  Borings   .         -.  .  129 

23.  At  the  Bottom  of  the  Great  Bore 140 

24.  Three  consecutive  Shells  of  the  Earth's  Crust    .         .  .  145 

25.  Earthquake  Routes  from  Japan  to  the  Isle  of  Wight     .  171 

26.  Showing   Coasts    invaded   by    the   Great    Sea-waves   from 

Krakatoa        .         .        V        .         .         .    *     .         .         .179 

27.  Spread  of  the  Air-wave  from  Krakatoa  to  the  Antipodes  183 

28.  The  great  Spiral  Nebula 193 

29.  How  to  find  the  great  Spiral  Nebula         ...         .         .196 

30.  A  group  of  Nebulae          .        .        .        .        .        ;        .       199 

xi 


Xii  LIST  OF  ILLUSTRATIONS 

FIG-  PACK 

31.  A  Ray  Nebula     .......  .  201 

32.  Portion  of  the  Milky  Way 205 

33.  A  Spiral  Nebula  seen  Edgewise 211 

34.  A  fore-shortened  Spiral 212 

35.  Edge-view  of  a  Spiral  boldly  shown 213 

36.  To  illustrate  Moment  of  Momentum       ....       223 

37.  Saturn 233 

38.  The  Ring  Nebula  in  Lyra 249 

39.  Lunar  Craters :    Hyginus  and  Albateginus         .         .         .  255 

40.  A  remarkable  Spiral 257 

41.  A  clearly-cut  Spiral 259 

42.  The  H  and  K  Lines  in  the  Photographic  Solar  Spectrum  276 

43.  Spectrum  of  Comet  showing  Carbon  Lines     .         .         .       290 

44.  Spectrum  of  the  Sun  during  Eclipse 291 

45.  A  Spiral  presented  Edgewise  ......       296 

46.  The  Plane  of  a  Planet's  Orbit 298 

47.  A  Right  Angle  divided  into  ten  parts     ....       301 

48.  Illustration  of  the  Second  Concord 309 

49.  Orbits  of  the  Earth,  Eros  and  Mars       ....       313 

50.  I.  A  Natural  System.     II.  An  Unnatural  System       .         .  318 

51.  An  elongated  irregular  Nebula         .....       329- 

52.  Two-branched  Spiral 345 

53.  Cluster  with  Stars  of  the  17th  magnitude      .         .         .       353 

54.  Spectrum  of  Neva  Persei   (1901) 359 

55.  The  Apteryx  ;    a  Wingless  Bird  of  New  Zealand    .         .       365 

56.  Skeleton  of  the  Apteryx,  showing  Rudimentary  Wings       .  366 

57.  Spirals   in  other   Departments  of  Nature :    Foraminifera  367 

58.  Ditto  ditto  Nautilus    .       367 

59.  To  illustrate  a  Theorem  in  the  Attraction  of  Gravitation  369 

60.  First    Law   of   Motion   exemplifies    Constant   Moment    of 

Momentum     .         .         .         .         .         .         .  •      •         .  375 

61.  A  useful  Geometrical  Proposition    .         .         .    .<-    .         .       376 

62.  Acceleration   of   Moment   of   Momentum    equals    Moment 

of  Force 376 

63.  Moment  of  Momentum  unaltered  by  Collision         .         .       380 


"  ~"^"^aa^ 

NIVERSITYJ 


Wtt}^' 

THE  EARTH'S  BEGINNING 


CHAPTER  I. 

INTRODUCTION. 

The  Earth's  Beginning— The  Nebular  Theory— Many  Applications  of 
the  Theory — The  Founders  of  the  Doctrine — Kant,  Laplace,  Will- 
iam Herschel :  Their  Different  Methods  of  Work — The  Vastness 
of  the  Problem— Voltaire's  Fable— The  Oak  Tree— The  Method 
of  Studying  the  Subject— Inadequacy  of  our  Time  Conceptions. 

I  TRY  in  these  lectures  to  give  some  account  of  an  excep- 
tionally great  subject — a  subject,  I  ought  rather  to  say, 
of  sublime  magnificence.  It  may,  I  believe,  be  affirmed 
without  exaggeration  that  the  theme  which  is  to  occupy 
our  attention  represents  the  most  daring  height  to  which 
the  human  intellect  has  ever  ventured  to  soar  in  its 
efforts  to  understand  the  great  operations  of  Nature. 
The  earth's  beginning  relates  to  phenomena  of  such  mag- 
nitude and  importance  that  the  temporary  concerns  which 
usually  engage  our  thoughts  must  be  forgotten  in  its 
presence.  Our  personal  affairs,  the  affairs  of  the  nation, 
and  of  the  empire — indeed,  of  all  nations  and  of  all  em- 
pires— nay,  even  all  human  affairs,  past,  present,  and  to 
come,  shrink  into  utter  insignificance  when  we  come  to 
consider  the  majestic  subject  of  the  evolution  of  that 
solar  system  of  which  our  earth  forms  a  part.  We  shall 
obtain  a  glimpse  of  what  that  evolution  has  been  in  the 


2  THE  EARTH'S  BEGINNING. 

mighty  chapter  of  the  book  of  Xature  on  which  we  are 
now  to  enter. 

The  nebular  theory  discloses  the  beginning  of  this- 
earth  itself.  It  points  out  the  marvellous  process  by 
which  from  original  chaos  the  firm  globe  on  which  we 
stand  was  gradually  evolved.  It  shows  how  the  founda- 
tions of  this  solid  earth  have  been  laid,  and  how  it  is  that 
we  have  land  to  tread  on  and  air  to  breathe.  But  the 
subject  has  a  scope  far  wider  than  merely  in  its  relation 
to  our  earth.  The  nebular  theory  accounts  for  the  be- 
ginning of  that  great  and  glorious  orb  the  sun,  which 
presides  over  the  system  of  revolving  planets,  guides  them 
in  their  paths,  illuminates  them  with  its  light,  and  stimu- 
lates the  activities  of  their  inhabitants  with  its  genial 
warmth.  The  nebular  theory  explains  how  it  comes 
about  that  the  sun  still  continues  in  these  latter  days  to 
shine  with  the  brilliance  and  warmth  that  it  had  through- 
out the  past  ages  of  human  history  and  the  vastly  greater 
periods  of  geological  time.  Then,  as  another  supreme 
achievement,  it  discloses  the  origin  of  the  planets  which 
accompany  the  sun,  and  shows  how  they  have  come  to  run 
their  mighty  courses;  and  it  tells  us  how  revolving  satel- 
lites have  been  associated  with  the  planets.  The  nebular 
theory  has,  indeed,  a  remarkable  relation  to  all  objects 
belonging  to  that  wonderful  scheme  which  we  call  the 
solar  system. 

It  should  also  be  noticed  that  the  nebular  theory  often 
brings  facts  of  the  most  diverse  character  into  striking 
apposition.  As  it  accounts  for  the  continued  mainte- 
nance of  the  solar  radiation,  so  it  also  accounts  for  that 
beneficent  rotation  by  which  each  continent,  after  the 


OF  THE 

UNIVERS 


I  T  Yj 
or  V 

THE 


NEBULAR   THEORY. 


enjoyment  of  a  day  under  the  invigorating  rays  of  the 
sun,  passes  in  due  alternation  into  the  repose  of  night. 
The  nebular  theory  is  ready  with  an  explanation  of  the 
marvellous  structure  revealed  in  the  rings  of  Saturn,, 
and  it  shows  at  the  same  time  how  the  volcanoes  of  the 
moon  acquired  their  past  phenomenal  activity,  and  whyr 
after  ages  of  activity,  they  have  now  at  last  become  ex- 
tinct. With  equal  versatility  the  nebular  theory  will 
explain  why  a  collier  experiences  increasing  heat  as  he 
descends  the  coalpit,  and  why  the  planet  Jupiter  is 
marked  with  those  belts  which  have  so  much  interest  for 
the  astronomer.  The  nebular  theory  offers  an  imme- 
diate explanation  of  the  earthquake  which  wrought  such 
awful  destruction  at  Lisbon,  while  it  also  points  out  the 
source  of  the  healing  warmth  of  the  waters  at  Bath. 
Above  all,  the  nebular  theory  explains  that  peerless  dis- 
covery of  cosmical  chemistry  which  declares  that  those 
particular  elements  of  which  the  sun  is  composed  are  no 
other  than  the  elements  which  form  the  earth  beneath 
our  feet. 

When  a  doctrine  of  such  transcendent  importance  i& 
proposed  for  our  acceptance,  it  is  fitting  that  we  should 
look,  in  the  first  instance,  to  the  source  from  which  the 
doctrine  has  emanated.  It  would  already  have  made  good 
its  claim  to  most  careful  hearing,  though  not  perhaps  to 
necessary  acceptance,  if  it  came  to  us  bearing  creden- 
tials which  prove  it  to  be  the  outcome  of  the  thought 
and  research  of  one  endowed  with  the  highest  order  of 
intellect.  If  the  nebular  theory  had  been  propounded 
by  only  a  single  great  leader  of  thought,  the  sublimity 
of  the  subject  with  which  it  deals  would  have  compelled 


4  THE  EARTH'S  BEGINNING. 

the  attention  of  those  who  love  to  study  the  book  of 
Nature.  If  it  had  appeared  that  a  second  investigator, 
also  famous  for  the  loftiest  intellectual  achievement, 
had  given  to  the  nebular  theory  the  sanction  of  his  name, 
a  very  much  stronger  claim  for  its  consideration  would 
at  once  have  been  established.  If  it  should  further  ap- 
pear that  yet  a  third  philosopher,  a  man  who  was  also 
an  intellectual  giant,  had  been  conducted  to  somewhat 
similar  conclusions,  we  should  admit,  I  need  hardly  say, 
that  the  argument  had  been  presented  with  still  further 
force.  It  may  also  be  observed  that  there  might  even 
be  certain  conditions  in  the  work  of  the  three  philoso- 
phers which  would  make  for  additional  strength  in  the 
cause  advocated;  if  it  should  be  found  that  each  of  the 
great  men  of  science  had  arrived  at  the  same  conclusion 
irrespective  of  the  others,  and,  indeed,  in  total  ignorance 
of  the  line  of  thought  which  his  illustrious  compeers 
were  pursuing;  this  would,  of  course,  be  in  itself  a  cor- 
roboration.  If,  finally,  the  methods  of  research  adopted 
by  these  investigators  had  been  wholly  different,  al- 
though converging  to  the  establishment  of  the  theory, 
then  even  the  most  sceptical  might  be  disposed  to  concede 
the  startling  claim  which  the  theory  made  upon  his  reason 
and  his  imagination. 

All  the  conditions  that  I  have  assumed  have  been 
fulfilled  in  the  presentation  of  the  nebular  theory  to  the 
scientific  world.  It  would  not  be  possible  to  point  to 
three  names  more  eminent  in  their  respective  branches 
of  knowledge  than  those  of  Kant,  Laplace,  and  William 
Herschel.  Kant  occupies  a  unique  position  by  the  pro- 
fundity and  breadth  of  his  philosophical  studies ;  Laplace 


THREE  GREAT  MEN.  5 

applied  the  great  discoveries  of  Newton  to  the  investiga- 
tion of  the  movements  of  the  heavenly  bodies,  publishing 
the  results  in  his  immortal  work,  Mecanique  Celeste; 
Herschel  has  been  the  greatest  and  the  most  original 
observer  of  the  heavens  since  the  telescope  was  invented. 
It  is  not  a  little  remarkable  that  the  great  philosopher 
from  his  profound  meditation,  the  great  mathematician 
from  a  life  devoted  to  calculations  about  the  laws  of 
Nature,  the  great  observer  from  sounding  the  depths  of 
the  firmament,  should  each  in  the  pursuit  of  his  own  line 
of  work  have  been  led  to  believe  that  the  grand  course  of 
Nature  is  essentially  expressed  by  the  nebular  theory. 
There  have  been  differences  of  detail  in  the  three  the- 
ories ;  indeed,  there  have  been  differences  in  points  which 
are  by  no  means  unimportant.  This  was  unavoidable  in 
the  case  of  workers  along  lines  so  distinct,  and  of  a  sub- 
ject where  many  of  the  data  were  then  unknown,  as 
indeed  many  are  still.  Even  at  the  present  day  no  man 
can  give  a  complete  account  of  what  has  happened  in 
the  great  evolution.  But  the  monumental  fact  remains 
that  these  three  most  sagacious  men  of  science,  whose 
lives  were  devoted  to  the  pursuit  of  knowledge,  each  ap- 
proaching the  subject  from  his  own  direction,  each  pur- 
suing his  course  in  ignorance  of  what  the  others  were 
doing,  were  substantially  led  to  the  same  result.  The 
progress  of  knowledge  since  the  time  when  these  great 
men  lived  has  confirmed,  in  ways  which  we  shall  en- 
deavour to  set  forth,  the  sublime  doctrine  to  which  their 
genius  had  conducted  them. 

Immanuel  Kant,  whose  grandfather  was  a  Scotsman, 
was  born  in  1724  at  Konigsburg,  where  his  life  was 
2 


6  THE  EARTH'S  BEGINNING. 

spent  as  a  professor  in  the  University,  and  where  he  died 
in  1804.  In  the  announcement  of  the  application  of 
the  principle  of  evolution  to  the  solar  system,  Laplace  was 
preceded  by  this  great  German  philosopher.  The  pro- 
found thinker  who  expounded  the  famous  doctrine  of 
time  and  space  did  not  disdain  to  allow  his  attention  to  be 
also  occupied  with  things  more  material  than  the 
subtleties  of  metaphysical  investigation.  As  a  natural 
philosopher  Kant  was  much  in  advance  of  his  time.  His 
speculations  on  questions  relating  to  the  operations  in 
progress  in  the  material  universe  are  in  remarkable  con- 
formity with  what  is  now  accepted  as  the  result  of 
modern  investigation.  Kant  outlined  with  a  firmness 
inspired  by  genius  that  nebular  theory  to  which  La- 
place subsequently  and  independently  gave  a  more 
definite  form,  and  which  now  bears  his  name. 

Kant's  famous  work  with  which  we  are  now  con- 
cerned appeared  in  1755.*  In  it  he  laid  down  the  im- 
mortal principle  of  the  nebular  theory.  The  greatness 
of  this  book  is  acknowledged  by  all  who  have  read  it, 
and  notwithstanding  that  the  progress  of  knowledge  has 
made  it  obvious  that  many  of  the  statements  it  contains 
must  now  receive  modification,  Kant's  work  contains  the 
essential  principle  affirming  that  the  earth,  the  sun,  the 
planets,  and  all  the  bodies  now  forming  the  solar  sys- 
tem did  really  originate  from  a  vast  contracting  nebula. 
In  later  years  Kant's  attention  was  diverted  from  these 

*  We  are  now  fortunately  able  to  refer  the  English  reader  to  the 
work  of  Professor  W.  Hastie,  D.D.,  entitled  "Kant's  Cosmogony," 
Glasgow,  1900.  Kant's  most  interesting  career  is  charmingly  described 
in  De  Quincey's  "  Last  Days  of  Immanuel  Kant." 


IMMANUEL  KANT. 

(From  an  old  print.) 


fat* 


8  THE  EARTH'S  BEGINNING. 

physical  questions  to  that  profound  system  of  philosophy 
with  which  his  name  is  chiefly  associated.  The  nebular 
theory  is  therefore  to  be  regarded  as  incidental  to  Kant's 
great  lifework  rather  than  as  forming  a  very  large  and 
important  part  of  it. 

At  the  close  of  the  last  century,  while  France  was  in 
the  throes  of  the  Revolution,  a  school  of  French  mathe- 
maticians was  engaged  in  the  accomplishment  of  a  task 
which  marked  an  epoch  in  the  history  of  human  thought. 
Foremost  among  the  mathematicians  who  devoted  their 
energies  to  the  discussion  of  the  great  problems  of  the 
universe  was  the  illustrious  Laplace.  As  a  personal 
friend  of  Napoleon,  Laplace  received  marked  distinc- 
tion from  the  Emperor,  who  was  himself  enough  of  a 
mathematician  to  be  able  to  estimate  at  their  true  value 
the  magnificent  results  to  which  Laplace  was  conducted. 

It  was  at  the  commencement  of  Kant's  career,  and 
before  his  great  lifework  in  metaphysics  was  under- 
taken, that  he  was  led  to  his  nebular  theory  of  the  solar 
system.  In  the  case  of  Laplace,  on  the  other  hand,  the 
nebular  theory  was  not  advanced  until  the  close  of  the 
great  work  of  his  life.  The  Mecanique  Celeste  had 
been  written,  and  the  fame  of  its  author  had  been  estab- 
lished for  all  time;  and  then  in  a  few  pages  of  a  subse- 
quent volume,  called  the  Systeme  du  Monde,  he  laid 
down  his  famous  nebular  theory.  In  that  small  space 
he  gave  a  wonderful  outline  of  the  history  of  the  solar 
system.  He  had  not  read  that  history  in  any  books  or 
manuscripts;  he  had  not  learned  it  from  any  ancient  in- 
scriptions; he  had  taken  it  direct  from  the  great  book  of 
Nature. 


DISCOVERT  BY  MATHEMATICS.  9 

Influenced  by  the  caution  so  characteristic  of  one 
whose  life  had  been  devoted  entirely  to  the  pursuit  of  the 
most  accurate  of  all  the  sciences,  Laplace  accompanied 
his  announcement  of  the  nebular  theory  with  becoming 
words  of  warning.  The  great  philosopher  pointed  out 
that  there  are  two  methods  of  discovering  the  truths  of 
astronomy.  Some  truths  may  be  discovered  by  observ- 
ing the  heavenly  bodies  with  telescopes,  by  measuring 
with  every  care  their  dimensions  and  their  positions,  and 
by  following  their  movements  with  assiduous  watchful- 
ness. But  there  is  another  totally  different  method 
which  has  enabled  many  remarkable  discoveries  to  be 
made  in  astronomy;  for  discoveries  may  be  made  by 
mathematical  calculations  which  have  as  their  basis  the 
numerical  facts  obtained  by  actual  observation.  This 
mathematical  method  often  yields  results  far  more  pro- 
found than  any  which  can  be  obtained  by  the  astron- 
omer's telescope.  The  pen  of  the  mathematician  is 
indeed  an  instrument  which  sometimes  anticipates 
revelations  that  are  subsequently  confirmed  by  actual 
observation.  It  is  an  instrument  which  frequently  per- 
forms the  highly  useful  task  of  checking  the  deductions 
that  might  too  hastily  be  drawn  from  telescopic  observa- 
tions. It  is  an  instrument  the  scope  of  whose  discoveries 
embraces  regions  immeasurably  beyond  the  reach  of  the 
greatest  telescope.  The  pen  of  the  mathematician  can 
give  us  information  as  to  events  which  took  place  long 
before  telescopes  came  into  existence — nay,  even  unnum- 
bered ages  prior  to  the  advent  of  man  on  this  earth. 

Laplace  was  careful  to  say  that  the  nebular  theory 
which  he  sketched  must  necessarily  be  judged  by  a 


10  THE  EARTH'S  BEGINNING. 

standard  different  from  that  which  we  apply  to  astro- 
nomical truths  revealed  by  telescopic  observation  or 
ascertained  by  actual  calculation.  The  nebular  theory, 
said  the  great  French  mathematician,  has  to  be  received 
with  caution,  inasmuch  as  from  the  nature  of  the  case  it 
cannot  be  verified  by  observation,  nor  does  it  admit  of 
proof  possessing  mathematical  certainty. 

A  large  part  of  these  lectures  will  be  devoted  to  the 
evidence  bearing  upon  this  famous  doctrine.  Let  it 
suffice  here  to  remark  that  the  quantity  of  evidence  now 
available  is  vastly  greater  than  it  was  a  hundred  years 
ago,  and  furthermore,  that  there  are  lines  of  evidence 
which  can  now  be  followed  which  were  wholly  undreamt 
of  in  the  days  of  Kant  and  Laplace.  The  particular 
canons  laid  down  by  Laplace,  to  which  we  have  just  re- 
ferred, are  perhaps  not  regarded  as  so  absolutely  bind- 
ing in  modern  days.  If  we  were  to  reject  belief  in 
everything  which  cannot  be  proved  either  by  the  testi- 
mony of  actual  eye-witnesses  or  by  strict  mathematical 
deductions,  it  would,  I  fear,  fare  badly  with  not  a  few 
great  departments  of  modern  science.  It  will  not  be 
necessary  to  do  more  at  present  than  just  to  mention,  in 
illustration  of  this,  the  great  doctrine  of  the  evolution  of 
life,  which  accounts  for  the  existing  races  of  plants  and 
animals,  including  even  man  himself.  I  need  hardly 
say  that  the  Darwinian  theory,  which  claims  that  man 
has  come  by  lineal  descent  from  animals  of  a  lower  type, 
admits  of  no  proof  by  mathematics;  it  receives  assuredly 
no  direct  testimony  from  eye-witnesses;  and  yet  the  fact 
that  man  has  so  descended  is,  I  suppose,  now  almost 
universally  admitted. 


WHAT  HER8GHEL  DID.  11 

In  the  case  of  the  great  German  philosopher,  as  well 
as  in  the  case  of  the  great  French  mathematician,  the 
enunciation  and  the  promulgation  of  their  nebular  the- 
ories were  merely  incidental  to  the  important  scientific 
undertakings  with  which  their  respective  lives  were 
mainly  occupied.  The  relation  of  the  nebular  theory 
to  the  main  lifework  of  the  third  philosopher  I  have 
named,  has  been  somewhat  different.  When  William 
Herschel  constructed  the  telescopes  with  which,  in  con- 
junction with  his  illustrious  sister,  he  conducted  his  long 
night-watches,  he  discovered  thousands  of  new  nebulae; 
he  may,  in  fact,  be  said  to  have  created  nebular  astron- 
omy as  we  now  know  it.  Ever  meditating  on  the  objects 
which  his  telescopes  brought  to  light,  ever  striving  to 
sound  the  mysteries  of  the  universe,  Herschel  perceived 
that  between  a  nebula  which  was  merely  a  diffused  stain 
of  light  on  the  sky,  and  an  object  which  was  hardly  dis- 
tinguishable from  a  star  with  a  slight  haze  around  it, 
every  intermediate  grade  could  be  found.  In  this  way 
he  was  led  to  the  splendid  discovery  which  announced 
the  gradual  transformation  of  nebulae  into  stars.  We 
have  already  noted  how  the  profound  mathematician  was 
conducted  to  a  view  of  the  origin  of  the  solar  system 
which  was  substantially  identical  with  that  which  had 
been  arrived  at  by  the  consummate  metaphysician.  The 
interest  is  greatly  increased  when  we  find  that  similar 
conclusions  were  drawn  independently  from  the  tele- 
scopic work  of  the  most  diligent  and  most  famous 
astronomical  observer  who  has  ever  lived.  Not  from 
abstract  speculation  like  Kant,  not  from  mathematical 
suggestion  like  Laplace,  but  from  accurate  and  laborious 


12  THE  EARTH'S  BEGINNING. 

study  of  the  heavens  was  the  great  William  Herschel 
led  to  the  conception  of  the  nebular  theory  of  evolu- 
tion. 

That  three  different  men,  of  science,  approaching  the 
study  of  perhaps  the  greatest  problem  which  Nature  of- 
fers us  from  points  of  view  so  fundamentally  different, 
should  have  been  led  substantially  to  the  same  result, 
is  a  remarkable  incident  in  the  history  of  knowledge. 
Surely  the  theory  introduced  under  such  auspices  and 
sustained  by  such  a  weight  of  testimony  has  the  very 
strongest  claim  on  our  attention  and  respect. 

In  the  discussion  on  which  we  are  about  to  enter 
in  these  lectures  we  must  often  be  prepared  to  make  a 
special  effort  of  the  imagination  to  help  us  to  realise 
how  greatly  the  scale  of  the  operations  on  which  the 
attention  is  fixed  transcends  that  of  the  phenomena  with 
which  our  ordinary  affairs  are  concerned.  Our  eyes  can 
explore  a  region  of  space  which,  however  vast,  must  still 
be  only  infinitesimal  in  comparison  with  the  extent  of 
space  itself.  Notwithstanding  all  that  telescopes  can  do 
for  us,  our  knowledge  of  the  universe  must  be  necessarily 
restricted  to  a  mere  speck  in  space,  a  speck  which  bears 
to  the  whole  of  space  a  ratio  less — we  might  perhaps  say 
infinitely  less — than  that  which  the  area  of  a  single  daisy 
bears  to  the  area  of  the  continent  where  that  daisy 
blooms.  But  we  need  not  repine  at  this  limitation;  a 
whole  life  devoted  to  the  study  of  a  daisy  would  not  be 
long  enough  to  explore  all  the  mysteries  of  its  life.  In 
like  manner  the  duration  of  the  human  race  would  not  be 
long  enough  to  explore  adequately  even  that  small  part  of 
space  which  is  submitted  for  our  examination. 


WHERE   WE  BEGIN.  13 

But  it  is  not  merely  the  necessary  limits  of  our  senses 
which  restrict  our  opportunities  for  the  study  of  the  great 
phenomena  of  the  universe.  Man's  life  is  too  short  for 
the  purpose.  That  our  days  are  but  a  span  is  the  com- 
monplace of  the  preacher.  But  it  is  a  commonplace 
specially  brought  home  to  us  in  the  study  of  the  nebular 
theory.  A  man  of  fourscore  will  allude  to  his  life  as  a 
long  one,  and  no  doubt  it  may  be  considered  long  in  re- 
lation to  the  ordinary  affairs  of  our  abode  on  earth;  but 
what  is  a  period  of  eighty  years  in  the  history  of  the 
formation  of  a  solar  system  in  the  great  laboratory  of  the 
universe  ?  Such  a  period  then  seems  to  be  but  a  trifle — 
it  is  nothing.  Eighty  years  may  be  long  enough  to  wit- 
ness the  growth  of  children  and  grandchildren;  but  it  is 
too  short  for  a  single  heartbeat  in  the  great  life  of  Nature. 
Even  the  longest  lifetime  is  far  too  brief  to  witness  a 
perceptible  advance  in  the  grand  transformation.  The 
periods  of  time  demanded  in  the  great  evolution  shadowed 
forth  by  the  nebular  theory  utterly  transcend  our  ordi- 
nary notions  of  chronology.  The  dates  at  which  su- 
preme events  occurred  in  the  celestial  evolution  are  im- 
measurably more  remote  than  any  other  dates  which 
we  are  ever  called  upon  to  consider  in  other  departments 
of  science.  The  time  of  the  story  on  which  we  are  to  be 
engaged  is  earlier,  far  earlier,  than  any  date  we  have  ever 
learned  at  school,  or  have  ever  forgotten  since.  The  in- 
cidents of  that  period  took  place  long  before  any  date  was 
written  in  figures — earlier  than  any  of  those  very  ancient 
dates  which  the  geologists  indicate  not  by  figures  indeed, 
but  by  creatures  whose  remains  imbedded  in  the  rocks 
suffice  to  give  a  character  to  the  period  referred  to.  The 


14  THE  EARTH'S  BEGINNING. 

geologist  will  specify  one  epoch  as  that  in  which  the 
fossilized  bone  of  some  huge  extinct  reptile  was  part  of 
a  living  animal;  he  may  specify  another  by  the  state- 
ment that  the  shell  of  some  beautiful  ammonite  was 
then  inhabited  by  a  living  form  which  swam  in  the 
warm  primeval  seas.  The  date  of  our  story  has  at 
least  this  much  certainty:  that  it  is  prior — immeasur- 
ably prior — to  the  time  when  that  marvellous  thing 
which  we  call  life  first  came  into  being. 

Voltaire  has  an  instructive  fable  which  I  cannot  re- 
sist repeating.  It  will  serve,  at  all  events,  to  bring  be- 
fore us  the  way  in  which  the  lapse  of  time  ought  to  be 
regarded  by  one  who  desires  to  view  the  great  operations 
of  Nature  in  their  proper  proportions.  He  tells  how 
an  inhabitant  of  the  star  Sirius  went  forth  on  a  voy- 
age of  exploration  through  the  remote  depths  of  space. 
In  the  course  of  his  travels  he  visited  many  other 
worlds,  and  at  length  reached  Saturn,  that  majestic 
orb,  which  revolved  upon  the  frontier  of  the  solar  sys- 
tem, as  then  known.  Alighting  on  the  ringed  globe 
for  rest  and  investigation,  the  Sirian  wanderer,  in  quest 
of  knowledge,  was  successful  in  obtaining  an  interview 
with  a  stately  inhabitant  of  Saturn  who  enjoyed  the 
reputation  of  exceptional  learning  and  wisdom.  The 
Sirian  hoped  to  have  some  improving  conversation  with 
this  sage  who  dwelt  on  a  globe  so  utterly  unlike  his 
own,  and  who  had  such  opportunities  of  studying  the 
majestic  processes  of  Nature  in  remote  parts  of  the 
universe.  He  thought  perhaps  they  might  be  able  to 
compare  instructive  notes  about  the  constitution  of  the 
suns  and  systems  in  their  respective  neighbourhoods. 


THE  BUTTERFLY  AND   THE  OAK-TREE.          15 

The  visitor  accordingly  prattled  away  gaily.  He  opened 
all  his  little  store  of  knowledge  about  the  Milky  Way, 
about  the  Great  Bear,  and  about  the  great  Nebula  in 
Orion;  and  then  pausing,  he  asked  what  the  Saturnian 
had  to  communicate  in  reply.  But  the  philosopher  re- 
mained silent.  Eagerly  pressed  to  make  some  response, 
the  grave  student  who  dwelt  on  the  frontier  globe  at  last 
said  in  effect :  "  Sirian,  I  can  tell  you  but  little  of  Nature. 
I  can  tell  you  indeed  nothing  that  is  really  worthy  of  the 
great  theme  which  Nature  proposes;  for  the  grand  opera- 
tions of  Nature  are  very  slow;  they  are  so  slow  that  the 
great  transformations  in  progress  around  us  would  have 
to  be  watched  for  a  very  long  time  before  they  could  be 
properly  understood.  To  observe  Nature  so  as  to  per- 
ceive what  is  really  happening,  it  would  be  necessary  to 
have  a  long  life;  but  the  lives  of  the  inhabitants  of 
Saturn  are  not  long;  none  of  us  ever  lives  more  than 
fifteen  thousand  years." 

Change  is  the  order  of  Nature.  Many  changes  no 
doubt  take  place  rapidly,  but  the  great  changes  by 
which  the  system  has  been  wrought  into  its  present 
form,  those  profound  changes  which  have  produced  re- 
sults of  the  greatest  magnificence  in  celestial  architect- 
ure are  extremely  slow.  We  should  make  a  huge  mis- 
take if  we  imagined  that  changes — even  immense  changes 
— are  not  in  progress,  merely  because  our  brief  day  is  too 
short  a  period  wherein  to  perceive  them. 

On  the  village  green  stands  an  oak-tree,  a  veteran 
which  some  say  dates  from  the  time  of  William  the 
Conqueror,  but  which  all  agree  must  certainly  have 
been  a  magnificent  piece  of  timber  in  the  days  of  Queen 


16  THE  EARTH'S  BEGINNING. 

Elizabeth.  The  children  play  under  that  tree  just  as 
their  parents  and  their  grandparents  did  before  them. 
A  year,  a  few  years,  even  a  lifetime,  may  show  no  ap- 
preciable changes  in  a  tree  of  such  age  and  stature.  Its 
girth  does  not  perceptibly  increase  in  such  a  period. 
But  suppose  that  a  butterfly  whose  life  lasts  but  a  day 
or  two  were  to  pass  his  little  span  in  and  about  this  vener- 
able oak.  He  would  not  be  able  to  perceive  any  changes 
in  the  tree  during  the  insignificant  period  over  which  his 
little  life  extended.  Not  alone  the  mighty  trunk  and 
the  branches,  but  even  the  very  foliage  itself  would  seem 
essentially  the  same  in  the  minutes  of  the  butterfly's  ex- 
treme old  age  as  they  did  in  the  time  of  his  life's  meridian 
or  at  the  earliest  moment  of  his  youth.  To  the  observa- 
tions of  a  spectator  who  viewed  it  under  such  ephemeral 
conditions  the  oak-tree  would  appear  steadfast,  and  might 
incautiously  be  deemed  eternal.  If  the  butterfly  could 
reflect  on  the  subject,  he  might  perhaps  argue  that  there 
could  not  be  any  change  in  progress  in  the  oak-tree,  be- 
cause although  he  had  observed  it  carefully  all  his  life  he 
could  not  detect  any  certain  alteration.  He  might  there- 
fore not  improbably  draw  the  preposterous  conclusion 
that  the  oak-tree  must  always  have  been  just  as  large  and 
just  as  green  as  he  had  invariably  known  it ;  and  he  might 
also  infer  that  just  as  the  oak-tree  is  now,  so  will  it  re- 
main for  all  time. 

In  our  study  of  the  heavens  we  must  strive  to  avoid 
inferences  so  utterly  fallacious  as  these  which  I  have  here 
tried  to  illustrate.  Let  it  be  granted  that  to  our  super- 
ficial view  the  sun  and  the  moon,  the  stars  and  the  con- 
stellations present  features  which  appear  to  us  as  eternal 


Fig.  2. — A  FAINT  DIFFUSED  NEBULOSITY  (n.g.c.  1499;  in  Perseus). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 


18  THE  EARTH'S  BEGINNING. 

as  the  bole  of  the  oak  seemed  to  the  butterfly.  But 
though  the  sun  may  seem  to  us  always  of  the  same 
size  and  always  of  the  same  lustre,  it  would  be  quite 
wrong  to  infer  that  the  lustre  and  size  of  the  sun  are 
in  truth  unchanging.  The  sun  is  no  more  unchang- 
ing than  the  oak-tree  is  eternal.  The  sun  and  the 
earth,  no  less  than  the  other  bodies  of  the  universe, 
are  in  process  of  a  transformation  no  less  astonishing 
than  that  wonderful  transformation  which  in  the  course 
of  centuries  develops  an  acorn  into  the  giant  of  the 
forest.  We  could  not  indeed  with  propriety  apply  to 
the  great  transformation  of  the  sun  the  particular  word 
growth;  the  character  of  the  solar  transformation  cannot 
be  so  described.  The  oak-tree,  of  course,  enlarges  with 
its  years,  while  the  sun,  on  the  other  hand,  is  becoming 
smaller.  The  resemblance  between  the  sun  and  the  oak- 
tree  extends  no  further  than  that  a  transformation  is 
taking  place  in  each.  The  rate  at  which  each  trans- 
formation is  effected  is  but  slow;  the  growth  of  the  oak 
is  too  slow  to  be  perceived  in  a  day  or  two;  the  contrac- 
tion of  the  sun  is  too  slow  to  be  appreciable  within  the 
centuries  of  human  history. 

"Whatever  the  butterfly's  observation  might  have  sug- 
gested with  regard  to  the  eternity  of  the  oak,  we  know 
there  was  a  time  when  that  oak-tree  was  not,  and  we 
know  that  a  time  will  come  when  that  oak-tree  will  no 
longer  be.  In  like  manner  we  know  there  was  a  time 
when  the  solar  system  was  utterly  different  from  the  so- 
lar system  as  we  see  it  now ;  and  we  know  that  a  time  will 
come  when  the  solar  system  will  be  utterly  different 
from  that  which  we  see  at  present.  The  mightiest 


THE  OAK-TREE'S  GROWTH. 


changes  are  most  certainly  in  progress  around  us.  "We 
must  not  deem  them  non-existent,  merely  because  they 
elude  our  scrutiny,  for  our  senses  may  not  be  quick 
enough  to  perceive  the  small  extent  of  some  of  these 
changes  within  our  limited  period  of  observation.  The 
intellect  in  such  a  case  confers  on  man  a  power  of  sur- 
veying Nature  with  a 
penetration  immeasur- 
ably beyond  that  af- 
forded by  his  organs 
of  sense. 

That  the  great  oak- 
tree  which  has  lived 
for  centuries  sprang 
from  an  acorn  no  one 
can  doubt ;  but  what  is 
the  evidence  on  which 
we  believe  this  to  have 
been  the  origin  of  a  Fig'  B-^™  CRA*  NEBULA  (n'g'c- 

0  1952;  m  Taurus.) 

veteran  of  the  forest 

(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

when  history  and  tra- 
dition are  both  silent?  In  the  absence  of  authentic 
documents  to  trace  the  growth  of  that  oak-tree  from  the 
beginning,  how  do  we  know  that  it  sprouted  from  an 
acorn?  The  only  reason  we  have  for  believing  that  the 
oak-tree  has  gone  through  this  remarkable  development 
is  deduced  from  the  observation  of  other  oak-trees.  We 
know  the  acorn  that  has  just  sprouted;  we  know  the 
young  sapling  as  thick  as  a  walking  stick;  we  know  the 
vigorous  young  tree  as  stout  as  a  man's  arm  or  as  his 
body;  we  know  the  tree  when  it  first  approaches  the  dig- 


20  THE  EARTH'S  BEGINNING, 

nity  of  being  called  timber;  we  can  therefore  observe 
different  trees  grade  by  grade  in  a  continuous  succession 
from  the  acorn  to  the  monarch  of  five  centuries.  No  one 
doubts  for  a  moment  that  the  growth  as  witnessed  in  the 
stages  exhibited  by  several  different  trees,  gives  a  sub- 
stantially accurate  picture  of  the  development  of  any  in- 
dividual tree.  Such  is  the  nature  of  one  of  the  argu- 
ments which  we  apply  to  the  great  problem  before  us. 
We  are  to  study  what  the  solar  system  has  been  in  the 
course  of  its  history  by  the  stages  which  we  witness  at  the 
present  moment  in  the  evolution  of  other  systems  through- 
out the  universe. 

The  mighty  transformation  through  which  the  solar 
system  has  passed,  and  is  even  now  at  this  moment 
passing,  cannot  be  actually  beheld  by  us  poor  creatures 
of  a  day.  It  might  perhaps  be  surveyed  by  beings  whose 
pulses  counted  centuries,  as  our  pulses  count  seconds, 
by  beings  whose  minutes  lasted  longer  than  the  dynasties 
of  human  history,  by  beings  to  whom  a  year  was  com- 
parable with  the  period  since  the  earth  was  young,  and 
since  that  wondrous  thing  we  call  life  began  to  move  in 
the  waters. 

May  I,  with  all  reverence,  try  to  attune  our  thoughts 
to  the  time-conceptions  required  in  this  mighty  theme 
by  quoting  those  noble  lines  of  the  hymn — 

"  A  thousand  ages  in  Thy  sight 

Are  like  an  evening  gone, 
Short  as  the  watch  that  ends  the  night, 
Before  the  rising  sun. " 


CHAPTER  II. 

THE   PROBLEM  STATED. 

The  G-reat  Diurnal  Motion  —  The  Distinction  between  Stars  and 
Planets — The  Earth  no  more  than  a  Planet — Relation  of  the  Stars 
to  the  Solar  System — Contrast  between  Aldebaran  and  Mars — Illus- 
tration of  Star-distances — The  Celestial  Perspective — Illustration  of 
an  Attractive  Force — Instructive  Experiments — The  Globe  and  the 
Tennis  Ball— The  Law  of  Gravitation— The  Focal  Ellipse— The  Solar 
System  as  it  is  now  Known — Statement  of  the  Great  Problem 
before  us. 

WHEN  we  raise  our  eyes  to  the  heavens  on  a  clear  night, 
thousands  of  bright  objects  claim  our  attention.  We 
observe  that  all  these  objects  move  as  if  they  were 
fastened  to  the  inside  of  an  invisible  sphere.  They  are 
seen  gradually  ascending  from  the  east,  passing  across 
the  south,  and  in  due  course  sinking  towards  the  west. 
The  sun  and  the  moon,  as  well  as  all  the  other  bodies, 
alike  participate  in  this  great  diurnal  movement.  The 
whole  scheme  of  celestial  objects  seems  to  turn  around 
the  two  points  in  the  heavens  that  we  call  the  Poles, 
and  so  far  as  the  pole  in  the  northern  hemisphere  is 
concerned,  its  position  is  most  conveniently  indicated 
by  the  proximity  of  the  well-known  Pole  Star. 

Except  this  great  diurnal  motion,  the  vast  majority 


'22  THE    EARTH'S   BEGINNING. 

of  the  bodies  on  the  celestial  sphere  have  no  other 
movement  directly  appreciable,  and  certainly  none 
which  it  is  necessary  for  us  to  consider  at  present. 
The  groups  in  which  the  stars  have  been  arranged  by 
the  poetical  imagination  of  the  ancients  exist  to-day,  as 
they  have  existed  during  all  the  ages  since  they  were 
first  recognised,  without  any  noticeable  alteration  in 
their  lineaments.  The  stately  belt  of  Orion  is  seen 
to-night  as  Job  beheld  it  thousands  of  years  ago ;  the 
Stars  in  the  Pleiades  have  not  altered  their  positions, 
relatively  to  the  adjacent  stars  nor  their  arrangement 
among  themselves,  since  the  time  when  astronomers  in 
early  Greece  observed  them.  All  the  bodies  which  form 
these  groups  are  therefore  known  as  fixed  stars. 

But  besides  the  fixed  stars,  which  exist  in  many 
thousands,  and,  of  course,  the  sun  and  the  moon,  there 
are  other  celestial  objects,  so  few  in  number  as  to  be 
counted  on  the  fingers  of  one  hand,  which  are  in  no 
sense  fixed  stars.  It  is  quite  true  that  these  wandering 
bodies,  or  planets,  as  they  are  generally  designated,  bear 
a  certain  resemblance  to  the  fixed  stars.  In  each  case 
the  star  or  the  planet  appears  as  a  bright  point,  like 
many  other  bright  points  in  the  heavens,  and  star  and 
planet  both  participate  in  the  general  diurnal  motion. 
But  a  little  attention  will  show  that  while  the  stars, 
properly  so  called,  retain  their  relative  places  for 
months  and  years  and  centuries,  the  planets  change 
their  places  so  rapidly  that  in  the  course  of  a  few 
nights  it  is  quite  easy  to  see,  even  without  the  aid  of 
any  instrument,  that  they  have  independent  motion. 

We  may  compare  the  movements  of  these  bodies  to 
the  movement  of  the  moon,  which  nightly  shifts  her 
place  over  a  long  track  in  the  sky;  and  although  we 


THE   PLANETS.  23 

are  not  able  to  see  the  stars  in  the  vicinity  of  the  sun, 
inasmuch  as  the  brilliant  light  of  the  orb  quenches  the 
feeble  radiance  from  such  stars,  there  is  no  doubt  that, 
did  we  see  them,  the  sun  itself  would  seem  to  move 
relatively  to  the  stars,  just  as  does  the  moon  and  just 
as  do  the  planets. 

The  distinction  among  the  heavenly  bodies  between 
stars  and  planets  was  noticed  by  acute  observers  of 
Nature  in  the  very  earliest  times.  The  names  of  the 
planets  come  to  us  as  survivals  from  the  time  when 
the  sun,  the  moon,  and  the  stars  were  objects  of 
worship,  and  they  come  to  us  bearing  the  names  of 
the  deities  of  which  these  moving  globes  were  regarded 
as  the  symbols.  But  it  was  not  the  movements  of  the 
planets  alone  which  called  for  the  notice  of  the  early 
observers  of  the  skies.  The  brightness  and  certain 
other  features  peculiar  to  them  also  attracted  the 
attention  of  the  primitive  astronomers.  They  could 
not  fail  to  observe  that  when  the  beautiful  planet 
Venus  was  placed  so  as  to  be  seen  to  the  greatest 
advantage,  her  orb  was  far  brighter  than  any  other 
object  in  the  host  of  heaven,  the  sun  and  the  moon 
both  of  course  excepted.  It  was  also  obvious  that 
Jupiter  at  his  best  exceeded  the  stars  in  lustre,  and 
sometimes  approached  even  to  that  of  Venus  itself. 
Though  Mercury  was  generally  so  close  to  the  sun 
as  to  be  invisible  among  its  beams,  yet  on  the  rare 
occasions  when  that  planet  was  seen,  just  after  sun- 
set or  just  before  sunrise,  its  lustre  was  such  as  to 
mark  it  out  as  one  of  the  remarkable  bodies  in  the 
heavens. 

Thus  the  astronomers  of  the  earliest  ages  pointed 
to  the  five  planets  and  the  sun  and  the  moon  as  the 


24  THE    EARTH'S    BEGINNING. 

seven  wandering  stars.  The  diligent  attention  of  the 
learned  of  every  subsequent  period  was  given  to  the 
discovery  of  the  character  of  their  movements.  The 
problems  that  these  motions  presented  were,  however, 
so  difficult  that  not  until  after  the  lapse  of  thousands 
of  years  did  their  nature  become  understood.  The 
supreme  importance  of  the  earth  appeared  so  obvious 
to  the  early  astronomers  that  it  did  not  at  first 
occur  to  them  to  assign  to  our  earth  a  position 
which  would  reduce  it  to  the  same  class  as  any  of 
the  celestial  bodies.  The  obviously  great  size  of  our 
globe,  the  fact  that  to  the  uninstructed  senses  the 
earth  seemed  to  be  at  rest,  while  the  other  bodies 
seemed  to  be  in  motion,  and  many  other  analogous 
circumstances,  appeared  to  show  that  the  earth  must 
be  a  body  totally  different  from  the  other  objects 
distributed  around  us  in  space.  It  was  only  by 
slow  degrees,  and  after  much  observation  and  reflec- 
tion, and  not  a  little  controversy,  that  at  last  the 
true  nature  of  our  system  was  detected.  Those  who 
have  been  brought  up  from  childhood  in  full  know- 
ledge of  the  rotation  of  the  earth  and  of  the  other 
fundamental  facts  relating  to  the  celestial  sphere, 
\\ill  often  find  it  difficult  to  realise  the  way  such 
problems  must  have  presented  themselves  to  the 
observers  of  old,  who  believed,  as  for  centuries  men 
did  believe,  that  the  earth  was  a  plane  of  indefinite 
extent  fixed  in  space,  and  that  the  sun  and  the 
planets,  the  moon  and  the  stars,  were  relatively  small 
bodies  whose  movements  must  be  accounted  for  as 
best  they  could  be,  consistently  with  the  fixity  and 
flatness  of  the  earth. 

But  at   last  it   began   to   be   seen   that   the   earth 


THE    EARTH    A    PLANET. 


25 


Fig.  4.— JUPITER  (May  30th,  1899,   lOh.  9.5m.,    g.m.t.). 
(E.  M.  Antoniadi.) 

must  be  relegated  to  a  position  infinitely  less  im- 
portant than  that  which  the  untutored  imagination 
assigned  to  it.  It  was  found  that  the  earth  was  not 
an  indefinite  plane ;  it  was  rather  a  globe  poised  in 
space,  without  direct  material  support  from  any  other 
body.  It  was  found  that  the  earth  was  turning 
round  on  its  axis :  while  instead  of  the  sun  revolving 
around  the  earth,  it  was  much  more  correct  to  say 
that  the  earth  revolved  around  the  sun.  The  as- 
tonishing truth  was  then  disclosed  that  the  five 
planets,  Jupiter  and  Saturn,  Mercury,  Venus  and  Mars, 
stood  in  a  remarkable  relation  to  the  earth.  For  as 
each  of  these  planets  was  found  to  revolve  round  the 


26  THE   EARTH'S   BEGINNING. 

sun,  and  as  the  earth  also  revolved  round  the  sun, 
the  assumed  difference  in  character  between  the  earth 
and  the  planets  tended  to  vanish  altogether.  There 
was  in  fact  no  essential  difference.  If  indeed  the 
earth  was  smaller  than  Jupiter  and  Saturn,  yet  it 
was  considerably  greater  and  heavier  than  Mars  or 
Mercury,  and  it  was  almost  exactly  the  same  size 
and  weight  as  Venus.  There  was  clearly  nothing  in 
the  question  of  bulk  to  indicate  any  marked  dif- 
ference between  our  earth  and  the  planets.  It  was 
also  observed  that  there  was  no  distinction  to  be 
drawn  between  the  way  in  which  the  earth  revolved 
round  the  sun  and  the  movements  of  the  planets. 
No  doubt  the  earth  is  not  so  near  the  sun  as 
Mercury ;  it  is  not  so  near  the  sun  as  even  Venus ;  on 
the  other  hand  the  sun  is  nearer  the  earth  than  Mars, 
while  Jupiter  is  a  long  way  further  off  than  Mars, 
and  Saturn  is  even  beyond  Jupiter  again.  It  is  these 
considerations  which  justify  us  in  regarding  our  earth 
as  one  of  the  planets.  We  have  also  to  note  the 
overwhelming  magnitude  of  the  sun  in  comparison 
with  any  one  of  the  planets.  It  will  suffice  to  give 
a  single  illustration.  The  sun  is  more  than  a 
thousand  times  as  massive  as  Jupiter,  and  Jupiter  is 
the  greatest  of  the  planets.  This  latter  noble  globe 
is  in  fact  greater  than  all  the  rest  of  the  planets 
put  together. 

But  before  we  can  fully  realise  the  circumstances 
of  the  solar  system,  it  will  be  necessary  to  see  how 
the  stars,  properly  so  called,  enter  into  the  scheme  of 
things  celestial.  The  stars  look  so  like  the  planets 
that  it  has  not  infrequently  happened  that  even  an 
experienced  astronomer  has  mistaken  one  for  the 


STAE8    AND    PLANETS.  27 

other.  The  planet  Mars  is  often  very  like  the  star 
Aldebaran,  and  there  are  not  a  few  first-magnitude 
stars  which  on  a  superficial  view  closely  resemble 
Saturn.  But  how  great  is  the  intrinsic  difference 
between  a  star  and  a  planet !  In  the  first  place  we 
have  to  note  that  every  planet  is  a  dark  object  like 
this  earth  of  ours,  possessing  no  light  of  its  own,  and 
dependent  entirely  on  the  sun  for  the  supply  of  light 
by  which  it  is  illumined.  But  a  star  is  totally  dif- 
ferent. The  star  is  not  a  dark  object,  but  is  really 
an  object  which  is  in  itself  intensely  luminous  and 
brilliant;  the  star  is  in  fact  a  sun-like  body.  How 
then,  it  may  well  be  asked,  does  a  star  like  Alde- 
baran, which  is  indeed  a  sun-like  body,  and  in  all 
probability  is  quite  as  large  and  quite  as  brilliant 
as  the  sun  itself,  bear  even  a  superficial  resemblance 
to  an  object  like  Mars,  which  would  not  be  visible 
at  all  were  it  not  for  the  illumination  with  which 
the  beams  from  the  sun  endow  it  ? 

The  explanation  of  this  striking  resemblance  is  to 
be  sought  in  the  relative  distances  of  the  two  objects. 
A  light  which  is  near  to  the  eye  may  produce  an 
effect  quite  as  great  as  a  very  much  stronger  light 
which  is  further  away.  The  intensity  of  a  light  varies 
inversely  as  the  square  of  the  distance.  If  the  distance 
of  a  light  from  the  eye  be  doubled,  then  the  intensity 
of  that  light  is  reduced  to  one-fourth.  Now  Aldebaran 
as  a  sun-like  body  emits  light  which  is  literally 
millions  of  times  as  great  as  the  gleam  of  sunshine 
which  starts  back  to  us  after  reflection  from  Mars; 
but  Aldebaran  is,  let  us  say,  a  million  times  as  far 
away  from  us  as  Mars,  and  this  being  so,  the  light 
from  Aldebaran  would  come  to  us  with  only  a  million- 


28  THE   EARTH'S   BEGINNING. 

millionth  part  of  the  intensity  that  it  would  have  if 
the  star  were  at  the  same  distance  as  the  planet. 
There  can  be  no  doubt  that  if  Aldebaran  were  merely 
at  the  same  distance  from  the  earth  as  Mars,  then 
Aldebaran  would  dispense  lustre  like  a  splendid  sun, 
By  moving  Aldebaran  further  off  its  light,  or  rather 
the  light  that  arrives  at  the  earth,  will  gradually  de- 
crease until  by  the  time  that  the  star  is  a  million 
times  as  far  as  Mars,  the  light  that  it  sends  us  is 
about  equal  to  that  of  Mars.  If  it  were  removed 
further  still,  the  light  that  it  would  send  us  would 
become  less  than  that  which  we  receive  from  Mars, 
and  if  still  more  remote,  Aldebaran  might  cease  to 
be  visible  altogether. 

This  illustration  will  suffice  to  explain  the  fun- 
damental difference  between  planets  and  stars,  not- 
withstanding the  fact  that  the  two  classes  of  bodies 
bear  to  each  other  a  resemblance  which  is  extremely 
remarkable,  even  if  it  must  be  described  as  being  in 
a  sense  accidental.  But  we  now  know  that  all 
of  the  thousands  of  stars  are  to  be  regarded  as  bril- 
liant suns,  some  of  which  may  not  be  so  far  off  as 
Aldebaran,  though  doubtless  some  are  very  much 
further.  The  actual  distances  are  immaterial,  for  the 
essential  point  to  notice  is  that  the  five  planets  are 
distinguished  from  the  stars,  not  merely  by  the  fact 
that  they  are  moving,  while  the  stars  are  at  rest,  but 
by  the  circumstance  that  the  planets  are  comparatively 
close  to  each  other  and  close  to  the  sun,  while  the 
stars  are  at  distances  millions  of  times  as  great  as 
the  distances  which  the  planets  are  from  each  other 
and  from  the  sun. 

We  are  now  enabled  to  place  the  scheme  of  things 


THE    SCALE    OF  THE    UNIVERSE.  2i> 

celestial  in  its  proper  perspective.  I  shall  suppose 
that  at  a  point  in  a  field  in  the  centre  of  England, 
somewhere  near  Leamington,  let  us  say,  we  drive  in 
a  peg  to  represent  the  sun.  Let  us  draw  a  circle 
with  that  peg  as  centre,  a  yard  being  the  radius,  and 
let  that  circle  represent  the  track  in  which  the  earth 
goes  round  the  sun.  I  do  not  indeed  say  that  the 
orbit  of  the  earth  is  exactly  a  circle,  and  the  actual 
shape  of  that  orbit  we  may  have  to  refer  to  later.  As, 
however,  the  apparent  size  of  the  sun  does  not  greatly 
alter  with  the  seasons,  it  is  evident  that  the  track 
which  our  earth  pursues  cannot  be  very  different 
from  a  circular  path.  Inside  this  circle  which  we 
have  drawn  with  a  yard  radius,  we  shall  put  two 
smaller  circles  which  are  to  represent  the  path  in 
which  Venus  moves,  and  the  path  in  which  Mercury 
moves.  Outside  the  path  of  the  earth  we  shall  draw 
another  circle  with  a  radius  of  five  yards;  this  will 
be  the  highway  along  which  the  majestic  Jupiter 
wends  his  way.  Inside  the  path  of  Jupiter  we  shall 
put  a  circle  which  will  represent  the  track  of  Mars, 
and  outside  the  path  of  Jupiter  a  circle  with  ten 
yards  as  radius  will  represent  the  track  of  Saturn. 
In  each  of  these  circles  we  shall  suppose  the  corre- 
sponding planet  to  revolve,  and  the  time  of  revolu- 
tion will  of  course  be  greater  the  further  the  planet 
is  from  the  sun.  To  complete  one  of  its  circuits  the 
earth  will  require  a  year,  Jupiter  twelve  years,  while 
Saturn,  which  in  the  ancient  astronomy  moved  on 
the  frontier  of  the  solar  system,  will  need  thirty  years 
to  accomplish  its  mighty  journey. 

We    have    thus    obtained    a    plan    of    the    solar 
system ;    but    now    we   should    like    to    indicate    the 


30  THE    EARTH'S    BEGINNING. 

positions  which  some  of  the  stars  are  to  occupy  on 
the  same  scale.  Let  us,  to  begin  with,  see  where  the 
very  nearest  fixed  star  is  to  be  placed.  We  may 
suppose  that  the  field  at  the  centre  of  England,  in 
which  our  little  diagram  has  been  constructed,  is  a 
large  one,  so  that  we  can  represent  the  places  of 
objects  which  are  ten  or  twenty  times  as  far  from  the 
sun  as  Saturn.  It  is,  however,  certain  that  no  actual 
field  would  be  large  enough  to  contain  within  its 
bounds  the  points  which  would  faithfully  represent 
the  positions  of  even  the  nearest  fixed  stars.  The 
whole  county  of  Warwick  would  not  be  nearly  big 
enough  for  this  purpose ;  indeed  we  may  say  that 
the  whole  of  England,  or  indeed  of  the  United  King- 
dom, would  not  be  sufficiently  extensive.  If  we  re- 
presented the  star  at  its  true  relative  distance,  it 
could  not  be  put  down  anywhere  within  the  bounds 
of  the  United  Kingdom;  the  nearest  object  of  this 
kind  would  have  to  be  far  away  out  on  the  continent 
of  Europe,  or  far  away  out  on  the  Atlantic  Ocean, 
far  away  down  near  the  equator,  or  far  away  up  near 
the  pole.  This  illustration  will  at  all  events  give 
some  notion  of  the  isolated  position  of  the  sun,  with 
the  planets  revolving  around  it,  in  relation  to  the 
rest  of  the  host  of  heaven. 

We  thus  learn  that  the  real  scheme  of  the 
universe  is  widely  different  from  that  which  a  super- 
ficial glance  at  the  heavens  would  lead  us  to  expect. 
We  are  now  able  to  put  our  system  into  its  proper 
perspective.  We  are  to  think  of  the  universe  as  con- 
sisting of  a  myriad  suns,  each  sun,  however,  being  so 
far  from  the  other  suns  that  viewed  from  any  one 
of  its  neighbours  it  appears  only  oi  star-like  insig- 


THE   LAWS    OF   THE   SOLAR   SYSTEM.          31 

mficance.  Let  us  fix  our  attention  on  one  of  these 
suns  in  space,  and  imagine  that  around  it,  and  com- 
paratively close  to  it,  there  are  a  number  of  small 
particles  in  revolution,  the  particles  being  illumined 
by  the  light  and  warmed  by  the  heat  of  the  central 
body  to  which  they  are  attached.  Viewed  from  one 
of  those  particles,  the  sun  to  which  they  belong  would 
doubtless  appear  as  a  great  and  glorious  orb,  while  a 
.glance  from  one  of  these  particles  to  any  of  the  other 
myriad  suns  in  space  will  show  these  orbs  reduced 
to  mere  points  of  stellar  light  by  reason  of  their 
enormous  distance.  This  sun  and  the  particles 
around  it,  by  which  of  course  we  shall  understand 
the  planets,  constitute  what  we  know  as  the  solar 
system.  .  This  illustration  may  suffice  to  show  the 
isolation  of  our  system  in  space,  and  that  isolation  is 
due  to  the  vast  distances  by  which  the  sun  and  its 
attendant  worlds  are  separated  from  the  myriads  of 
other  bodies  which  form  the  sidereal  heavens.  We 
must  next,  so  far  as  our  present  subject  requires 
it,  consider  the  laws  according  to  which  the  planets 
belonging  to  that  system  revolve  around  the  sun. 

Let  us  think  first  of  a  single  one  of  these  bodies 
which,  as  is  most  natural,  we  shall  take  to  be  the 
earth  itself,  and  now  let  us  consider  by  what  agency 
the  movement  of  the  earth  around  the  sun  is  guided 
along  the  path  which  so  closely  resembles  a  circle. 
It  must,  of  course,  be  borne  in  mind  that  there  can 
be  no  direct  material  connection  between  the  two 
bodies ;  there  is  no  physical  bond  uniting  the  earth  to 
the  sun.  It  is,  however,  certain  that  some  influence 
proceeding  from  the  sun  does  really  control  the 
motion.  We  may  perhaps  illustrate  what  takes  place 


32  THE   EARTH'S    BEGINNING. 

in  the  following  manner.  Here  is  a  globe,  and  here 
in  my  hand  I  hold  a  tennis  ball,  which  is  attached 
to  a  silken  thread,  the  other  end  of  the  thread  being 
attached  to  the  ceiling.  The  tennis  ball  is  to  hang 
so  that  both  globe  and  ball  are  about  the  same 
height  from  the  floor.  We  put  the  globe  directly 
underneath  the  point  on  the  ceiling  from  which  the 
silken  thread  hangs.  If  I  draw  the  tennis  ball  aside 
and  simply  release  it,  then  of  course  everybody  knows 
what  happens — it  is  hardly  necessary  to  try  the  ex- 
periment— the  tennis  ball  falls  at  once  towards  the 
globe  and  strikes  it.  We  may,  if  we  please,  regard 
that  tendency  of  the  tennis  ball  towards  the  globe  as 
a  sort  of  attraction  which  the  globe  exercises  upon 
the  ball.  I  must,  however,  say  that  this  is  not  a 
strictly  accurate  version  of  what  actually  takes  place. 
The  attraction  of  the  earth  for  the  tennis  ball  is  of 
course  largely  neutralised  by  the  support  given  by 
the  silk  thread.  There  is  thus  only  a  slight  out- 
standing component  of  gravitation  acting  on  the  ball, 
and  this  component,  which  is  virtually  the  effective 
force  on  the  ball,  tends  to  draw  the  ball  directly  to- 
wards the  globe.  For  the  purpose  of  our  illustration 
we  may  neglect  the  direct  attraction  of  the  earth 
altogether ;  we  may  omit  all  thought  of  the  tension 
of  the  silken  thread.  If  there  were  indeed  no  attrac- 
tion from  the  earth,  the  tennis  ball  might  remain 
poised  in  space  without  falling ;  and  if  it  were  then 
attracted  by  the  globe  it  would  fly  towards  the  globe 
just  as  we  actually  see  it  do.  We  are  therefore 
justified  in  regarding  the  movement  of  the  tennis 
ball  as  equivalent  to  that  which  would  be  produced 
if  an  attractive  virtue  resided  in  the  globe  by  which 


.  5.— NEBULOUS  REGION  AND  STAR  CLUSTER  (n.g.c.  2237-9  in 
Monoceros) . 

(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 


34  THE   EARTH'S    BEGINNING. 

it  pulled  the  tennis  ball.  We  may  also  imagine  that 
the  globe  attracts  the  tennis  ball  in  all  its  positions; 
for  whatever  be  the  point  at  which  the  ball  is  re- 
leased it  starts  off  straight  towards  the  globe.  This 
is  our  first  experiment  in  which,  having  withdrawn  the 
ball,  it  is  merely  released  without  receiving  an  initial 
impulse  to  one  side. 

Let  us  now  try  a  different  experiment.  We  withdraw 
the  ball,  and,  instead  of  merely  releasing  it  quietly 
and  allowing  it  to  drop  directly  to  the  globe,  we  give 
it  a  little  throw  sideways,  perpendicular  to  the  line 
joining  it  to  the  centre  of  the  globe.  If  we  start  it 
with  the  proper  speed,  which  a  few  trials  will  indicate, 
the  ball  can  be  made  actually  to  move  in  a  circle 
round  the  globe.  If  the  initial  speed  be  somewhat 
different,  the  path  in  which  the  tennis  ball  moves  will 
not  be  a  circle ;  it  will  rather  be  an  ellipse  of  some 
form.  Even  if  the  speed  be  correct  the  orbit  will 
always  be  an  ellipse  if  the  direction  of  the  initial 
throw  be  not  perpendicular  to  the  line  joining  the 
ball  to  the  centre  of  the  globe.  We  can  make  the 
ball  describe  a  very  long  ellipse  or  an  ellipse  which 
differs  but  little  from  a  circle.  But  I  would  ask  you 
to  note  particularly  that,  no  matter  how  we  may  start 
the  tennis  ball  into  motion,  it  will,  so  long  as  it  passes 
clear  of  the  globe,  move  in  an  ellipse  of  some  kind ; 
but  in  making  this  statement  we  assume  that  a  circle 
is  a  particular  form  of  the  ellipse. 

And  now  for  the  lesson  which  we  are  to  learn 
from  this  experiment,  which,  as  it  is  so  easily  per- 
formed, I  would  wish  everyone  to  try  for  himself 
We  have  in  this  simple  device  an  illustration  of  the 
movement  of  a  planet  around  the  sun.  We  see  that 


THE   PLANETS    AND    THE    SUN.  35 

this  tennis  ball  can  be  made  to  move  in  a  circle 
round  the  globe,  and  that  as  it  performs  this  circular 
movement  the  globe  is  all  the  time  attracting  the 
ball  towards  it.  Thus  we  illustrate  the  important  law 
that  when  one  body  moves  round  another  in  a  cir- 
cular path  this  movement  takes  place  in  consequence 
of  a  force  of  attraction  constantly  exerted  between  the 
large  body  in  the  centre  and  the  body  revolving 
round  it. 

The  principle  here  involved  will  provide  the  ex- 
planation of  the  movements  of  the  planets  round 
the  sun.  Each  of  the  planets  revolves  round  the  sun 
in  an  orbit  which  is  approximately  circular,  and  each 
of  the  planets  performs  that  movement  because  it  is 
continually  attracted  by  the  sun.  It  is,  however,  neces- 
sary to  add  that  there  is  a  fundamental  difference 
between  the  attraction  of  the  sun  for  the  planets  and 
the  attraction  which  the  globe  appeared  to  exert  on 
the  tennis  ball  in  our  experiment.  The  difference 
relates  to  the  character  of  the  forces  in  the  two  cases. 
If  the  tennis  ball  be  drawn  but  a  very  small  distance 
from  the  globe,  the  attraction  between  the  two  bodies 
is  very  slight.  If  the  tennis  ball  be  drawn  to  a 
greater  distance  from  the  globe,  the  attraction  is 
increased  correspondingly  ;  and,  indeed,  in  this  experi- 
ment the  attraction  between  the  two  bodies  increases 
with  the  distance,  and  is  said  to  be  proportional  to 
the  distance. 

But  the  case  is  very  different  in  that  particular  kind 
of  attraction  by  which  the  sun  controls  the  movements 
of  the  planets.  This  attraction  of  gravitation,  as  it  is 
called,  also  depends  on  the  distance  between  the  two 
bodies.  But  the  attraction  does  not  increase  when  the 


36  THE    EARTH'S    BEGINNING. 

distance  of  the  two  bodies  increases,  tor  the  change  lies 
the  other  way.  The  attraction,  in  fact,  diminishes  more 
rapidly  than  the  distance  increases.  If  the  distance 
between  the  sun  and  a  planet  be  doubled,  then  the 
attraction  between  the  two  bodies  is  only  a  fourth  of 
what  the  attraction  was  between  the  two  bodies  in  the 
former  case.  This  difference  between  the  law  of  attrac- 
tion as  it  exists  in  the  solar  system  and  the  law  of 
attraction  which  is  exemplified  in  our  little  experiment 
produces  a  remarkable  contrast  in  the  resulting  move- 
ments. The  orbit  in  each  case  is,  no  doubt,  an  ellipse, 
but  in  the  case  of  the  tennis  ball  revolving  round  the 
globe  the  ellipse  is  so  circumstanced  that  the  fixed 
attracting  body  stood  at  its  centre,  while  in  the  case  of  a 
planet  revolving  round  the  sun  the  conditions  are  not  so 
simple.  The  sun  does  not  stand  in  the  centre  of  the 
ellipse.  The  sun  is  placed  at  that  remarkable  point  of 
the  ellipse  so  dear  to  the  heart  of  the  geometer,  which 
he  calls  the  focus. 

The  solar  system  consists,  first,  of  the  great  regu- 
lating orb,  the  sun ;  then  of  the  planets,  each  of  which 
revolves  in  its  own  track  round  the  sun ;  each  of  these 
tracks  is  an  ellipse,  and  all  these  ellipses  have  this  in 
common,  that  a  focus  in  each  is  identical  with  the 
centre  of  the  sun.  In  other  respects  the  ellipses  may  be 
quite  different.  To  begin  with,  they  are  not  in  the  same 
plane,  though  it  is  most  important  to  notice,  as  we  shall 
have  to  discuss  more  fully  hereafter,  that  these  planes 
are  not  very  much  separated.  The  dimensions  of  the 
ellipses  vary,  of  course,  for  the  different  planets,  and 
the  periods  that  the  planets  require  for  their  several 
revolutions  are  also  widely  different  in  the  cases  of 
the  different  bodies ;  for  the  greater  the  diameter  of 


THE    BODIES   IN   THE    SOLAR    SYSTEM.         37 

a  planet's  orbit,  the  longer  is  the  time  required  for 
that  planet  to  complete  a  single  journey  round  the 
sun.  The  sun  presiding  at  the  common  focus  of  the 
orbits  while  governing  the  planets  by  its  attraction, 
at  the  same  time  that  it  illumines  them  with  its 
light  and  warms  them  by  its  rays,  gives  the  concep- 
tion of  the  solar  system. 

But  the  planetary  system  I  have  here  indicated  is 
merely  that  system  as  known  to  the  ancients.  It  is 
very  imperfect  from  the  standpoint  of  our  present 
knowledge.  The  solar  system  as  we  now  know  it,  when 
telescopes  have  been  applied  with  such  marvellous 
diligence  and  success  to  the  discovery  of  new  bodies, 
is  a  system  of  much  greater  complexity.  To  the  five 
old  planets  have  been  added  two  new  and  majestic 
planets — Uranus  and  Neptune — which  revolve  outside 
the  track  of  Saturn.  Hundreds  of  smaller  planets, 
invisible  to  the  unaided  eye,  the  asteroids  as  they  are 
called,  also  describe  their  ellipses  round  the  presiding 
luminary.  And  then  just  as  the  sun  controls  the  planets 
revolving  round  it,  so  do  many  of  the  planets  them- 
selves preside  over  subordinate  systems  of  revolving 
globes.  Our  earth  has  a  single  attendant,  the  moon, 
which,  under  the  guidance  of  the  earth's  attraction, 
performs  its  monthly  journey  ;  Jupiter  has  its  -five 
moons,  while  Mars  has  two,  and  Saturn  eight  or  nine, 
besides  his  incomparable  system  of  rings,  and  we  must 
also  add  that  Uranus  has  four  satellites  and  Neptune 
one.  To  complete  the  tale  of  bodies  in  the  solar 
system,  we  should  add  many  thousands  of  comets,  not 
to  mention  their  more  humble  associates  the  meteors, 
which  swarm  in  countless  myriads.  Finally,  we  are 
to  remember  that  this  elaborate  system  associated  with 
4 


38  THE   EAETH'S   BEGINNING. 

the  sun  is  an  isolated  object  in  the  universe ;  it  is  but 
as  a  grain  of  sand  in  the  extent  of  infinite  space. 

As  we  contemplate  a  system  so  wonderful,  the 
question  naturally  arises,  How  came  that  system  into 
being  ?  We  have  to  consider  whether  the  laws  of 
nature  as  we  know  them  afford  any  rational  explana- 
tion of  the  manner  in  which  this  system  came  into 
existence,  any  rational  explanation  of  how  the  sun 
came  to  shine,  how  the  earth  had  its  beginning,  how 
the  planets  came  to  revolve  round  the  sun,  and  to 
rotate  on  their  own  axes.  We  have  to  seek  for  a 
rational  explanation  of  the  rings  of  Saturn,  and  of 
the  satellites  by  which  so  many  planets  are  attended. 
We  have  to  show  that  a  satisfactory  explanation  of 
these  remarkable  phenomena  is  forthcoming,  and  that 
it  is  provided  by  the  famous  doctrine  of  evolution, 
which  it  is  the  object  of  these  lectures  to  discuss. 


CHAPTER    III. 

THE   FIRE-MIST. 

Evolution  of  other  Bodies  in  the  Universe  —  The  Nebulae —  Esti- 
mate of  the  Size  of  the  Great  Nebula  in  Orion— Photograph 
of  that  Nebula  taken  at  Lick  Observatory  —  The  Dumb-bell 
Nebula— The  Crossley  Kenector— The  late  Professor  Keeler— 
Astonishing  Discovery-  of  New  Nebulae — 120,000  Nebulae — The 
Continuous  Chain  from  a  Fluid  Haze  of  Light  to  a  Star  —  The 
Celestial  Evolution. 

WE  commence  this  chapter  with  a  scrutiny  of  the 
heavens,  to  see  whether,  among  the  bodies  which  it 
contains,  we  can  discover  any  which  appear  at  this 
moment  to  be  in  the  condition  through  which  our 
system  has  passed  in  some  of  its  earlier  stages. 

So  far  as  our  unaided  vision  is  concerned,  we  can 
see  little  or  nothing  in  the  skies  which  will  render 
us  assistance  in  our  present  endeavour.  The  objects 
that  we  do  see  in  thousands  are,  of  course,  the  stars, 
and,  as  we  have  already  pointed  out,  the  stars  are 
sun-like  objects,  and  as  such  have  advanced  many 
stages  beyond  the  elementary  condition.  The  stars 
are  therefore  not  immediately  available  for  the  illus- 
tration we  require.  But  when  we  come  to  look  at 
the  heavens  through  our  telescopes  we  presently  find 


40  TEE    EARTH'S   BEGINNING. 

that  there  are  objects  which  were  not  visible  to  the 
eye,  and  which  are  neither  stars  nor  planets.  Closer 
examination  of  these  objects  with  the  powerful  in- 
struments of  modem  observatories,  and  especially 
with  the  help  of  those  marvellous  appliances  which 
have  enabled  us  to  learn  the  actual  chemistry  of 
the  heavenly  bodies,  supplies  the  suggestions  that  are 
required. 

For  not  only  does  the  telescope  reveal  myriads  of 
stars  which  the  naked  eye  cannot  detect;  not  only 
does  it  reveal  wonderful  clusters  in  which  thousands 
of  stars  are  grouped  closely  together  so  as  to  form 
spectacles  of  indescribable  magnificence,  when  we  take 
into  account  the  intrinsic  splendour  of  each  starlike 
point,  but  it  also  reveals  totally  different  objects, 
known  as  nebulae.  These  objects  are  not  stars  and 
are  not  composed  of  stars,  but  are  vast  extensions  of 
matter  existing  in  a  far  more  elementary  condition. 
It  is  to  these  curious  bodies  that  we  invite  special 
attention  at  present.  It  is  believed  that  they  offer 
a  remarkable  illustration  of  the  origin  of  the  solar 
system.  We  shall  first  consider  the  best  known  object 
of  this  class.  It  is  the  Great  Nebula  in  Orion. 

And  here  it  may  be  well  to  give  an  estimate  which 
which  will  enable  us  to  form  some  notion  of  the  size 
of  this  object.  We  are  accustomed  to  recognise  the 
stars  as  presenting  the  appearance  of  mere  points  of 
light;  but  an  object  like  the  Great  Nebula  stretches 
over  a  wide  area  of  the  sky.  As  to  the  actual  extent 
of  the  space  which  it  occupies  we  cannot  speak  with 
confidence.  The  fact  is  that  with  every  increase  in 
the  power  of  the  telescope  the  nebula  appears  to 
encroach  more  and  more  on  'the  darkness  of  space 


THE    ORE  AT   NEBULA    IN   ORION.  41 

around.  We  give  in  Fig.  6  a  representation  of  the 
Great  Nebula  as  it  appears  on  a  photographic  plate 
obtained  at  the  Lick  Observatory  in  California.  But 
no  picture  can  adequately  represent  the  extraordinary 


Fig.  6. — THE  GREAT  NEBULA  IN  ORION  (Lick  Observatory,  California). 

(From  the  Royal  Astronomical  Society  Series.) 

delicacy  of  the  object  and  the  softness  and  tender- 
ness with  which  the  blue  nebulous  light  fades  into 
the  black  sky  around.  And  it  must  not  be  imagined 
that  the  nebula,  as  seen  on  this  picture,  represents 
the  utmost  limits  of  the  object  itself.  Every  pro- 
longation of  the  exposure,  every  increase  in  the 


42  THE    EARTH'S   BEGINNING. 

sensitiveness  of  the  plate,  show  more  and  more  the 
extent  of  the  nebula. 

We  shall,  I  doubt  not,  still  be  within  the  bounds 
of  truth  if  we  say  that  the  nebula  extends  over 
an  area  ten  times  as  great  as  that  represented  in 
this  photograph.  I  shall,  however,  take  the  area  of 
the  object  as  shown  in  the  photograph  for  the 
purpose  of  our  calculation.  Let  us  say  that  the 
nebula,  as  it  is  here  represented,  covers  about  two 
degrees  square.  I  shall  not  attempt  to  express  in 
miles  the  dimensions  of  an  object  so  vast.  I  will  try 
to  give  a  conception  of  the  size  of  the  Great  Nebula 
in  a  different  manner.  Let  us  employ  the  dimensions 
of  our  solar  system  for  the  purpose  of  comparison. 
Let  us  suppose  that  we  draw,  upon  the  scale  of  this 
celestial  photograph,  a  map  which  shall  represent  the 
sun  in  the  centre,  the  earth  at  her  proper  distance 
from  the  sun,  and  Jupiter  in  his  orbit,  which  is  five 
times  the  diameter  of  the  earth's  orbit;  and  then  let 
us  mark  the  other  planets  at  their  respective  dis- 
tances, even  to  Neptune  revolving  in  his  great  ellipse, 
with  a  diameter  thirty  times  that  of  the  earth's  orbit. 
Let  us  then  take  the  area  of  the  orbit  described  by 
Neptune  as  a  unit  with  which  to  measure  the  size  of 
the  Great  Nebula  in  Orion.  We  shall  certainly  be 
well  within  the  actual  truth  if  we  say  that  a  million 
circles  as  big  as  that  described  by  Neptune  would 
not  suffice  to  cover  the  area  that  is  represented  on 
this  photograph.  .This  will  give  some  idea  of  the 
imposing  dimensions  of  the  Great  Nebula  in  Orion. 

But  I  would  not  have  it  to  be  supposed  that  the 
Great  Nebula  in  Orion  is  unique,  unless  in  respect  to  its 
convenient  position.  The  circumstances  of  its  situa- 


THE   DUMB-BELL  NEBULA.  43 

tion  in  space  happen  to  make  it  a  comparatively  easy 
object  for  observation  by  dwellers  on  the  earth.  There 
are,  however,  very  many  other  nebulae,  although,  with 
one  exception — namely,  the  Great  Nebula  in  Andro- 
meda, to  which  we  shall  have  to  refer  in  a  later 
chapter — they  do  not  from  our  point  of  observation 
appear  to  be  so  brilliant  as  the  nebula  in  Orion.  The 
fact  is  that  by  large  and  powerful  telescopes  multi- 
tudes of  these  nebulae  are  revealed,  and  the  number 
ever  tends  to  increase  as  greater  depths  in  space 
are  sounded.  Many  of  the  nebulas  are  objects  which 
possess  sufficient  detail  to  merit  the  particular  atten- 
tion which  they  receive  from  astronomers.  It  must, 
however,  be  confessed  that  by  far  the  greater  number 
of  these  objects  are  so  dimly  discerned  that  it  is  im- 
possible to  study  their  individual  characteristics. 

Among  the  nebulae  which  possess  sufficient  indi- 
viduality to  merit  study  for  our  present  purpose,  I 
must  mention  the  so-called  Dumb-bell.  This  most 
interesting  object  can  be  seen  in  any  good  telescope. 
It  requires,  however,  as  indeed  do  all  such  objects,  an 
instrument  of  the  highest  power  to  do  it  justice ; 
and  I  believe  the  best  picture  ever  obtained  of  this 
nebula  is  contained  in  a  photograph  taken  at  the  Lick 
Observatory  (Fig.  7).  I  may  take  this  opportunity 
of  mentioning  that  a  photograph  really  shows  more 
details  in  the  nebula  than  can  be  perceived  even  by 
the  most  experienced  eye  when  applied  to  the  most 
powerful  telescope  placed  in  the  most  favoured  situa- 
tion as  to  climate.  Those  lovers  of  nature  who  desire 
to  observe  celestial  objects  through  a  great  telescope, 
and  have  not  the  opportunity  of  gratifying  their 
wishes,  may  perhaps  derive  consolation  from  the  fact 


44  THE    EARTH'S   BEGINNING. 

that  a  good  photograph  actually  represents  the  object 
much  better  than  any  eye  can  see  it.  More  of  the 
nebula  is  to  be  seen  by  looking  at  the  photograph 
than  has  actually  been  directly  observed  by  the  eye 
of  any  astronomer. 

We  have  chosen  the  Dumb-bell  and  the  Great 
Nebula  in  Orion  as  characteristic  examples  of  this 
remarkable  class  of  celestial  objects ;  but  there  are 
many  others  to  which  I  might  refer,  some  of  which 
we  represent  in  these  pages.  The  Crab  Nebula  (Fig.  3) 
and  others  have  been  distinguished  by  special  names; 
but  I  must  forbear  to  dwell  further  on  them,  and 
rather  hasten  to  give  the  results  of  recent  observations 
which  have  enormously  extended  our  knowledge  of  the 
nebulous  bodies  in  the  universe. 

Let  me  first  explain  the  source  whence  this 
extraordinary  accession  to  our  knowledge  has  arisen. 
We  owe  it  to  the  astronomers  at  the  Lick  Observatory, 
that  remarkable  institution  placed  on  the  summit  ot 
Mount  Hamilton  in  California.  Many  important  dis- 
coveries had  already  been  made  with  the  noble 
instruments  with  which  the  famous  Lick  Observatory 
had  originally  been  endowed  by  its  founder;  it  is, 
however,  by  a  recent  addition  to  its  magnificent  ap- 
paratus that  the  discoveries  have  been  made  which 
are  specially  significant  for  our  present  purpose. 

Many  years  ago  Dr.  A.  A.  Common,  the  distinguished 
English  astronomer,  constructed  an  exquisite  reflecting 
telescope  of  three  feet  aperture  (Fig.  8).  With  this  tele- 
scope Dr.  Common  himself  obtained  notable  results  in 
photographing  the  heavens,  and  his  success  earned  the 
award  of  the  Gold  Medal  of  the  Royal  Astronomical 
Society.  This  telescope  passed  into  the  possession  of 


PROFESSOR   KEELER. 


45 


Fig.  7. — THE  DUMB-BELL  NEBULA  (Lick  Observatory,  California). 
(From  the  Royal  Astronomical  Society  Series.) 


Mr.  E.  Crossley,  of  Halifax,  and  some  time  later  Mr. 
Crossley  presented  it  to  the  Lick  Observatory.  The 
great  mirror,  after  its  voyage  across  the  Atlantic,  was 
duly  erected  on  the  top  of  Mount  Hamilton,  and 
fortunately  for  science  Professor  Keeler,  whose  early 
death  astronomers  of  both  continents  greatly  deplore, 
devoted  himself  to  the  study  of  the  heavens  with  its 
aid.  He  encountered  many  difficulties,  as  might  per- 
haps be  expected  in  such  a  task  as  he  proposed.  His 
patience  and  skill,  however,  overcame  them,  and 
though  death  terminated  his  labours  when  his  great 
programme  had  but  little  more  than  commenced,  the 
work  he  had  already  accomplished  has  led  to  results 
of  the  most  striking  character.  Of  the  skill  that  he 
obtained  in  photographing  celestial  nebulae  we  have 
given  illustrations  in  Figs.  6  and  7. 


46  THE   EARTH'S    BEGINNING. 

It  is  not  to  the  individual  portraits  of  notable 
nebulae  that  we  are  now  about  to  refer.  The  most  strik- 
ing characteristic  of  the  sidereal  heavens  is  not  to  be 
found  in  the  fact  that  in  one  part  of  the  sky  we  have 
a  brilliant  Sirius,  in  another  a  Capella,  and  in  a  .third 
a  Canopus,  but  in  the  fact  that  the  heavens  wherever 
we  may  test  them  are  strewn  with  incalculable 
myriads  of  stars,  many  of  which  appear  faint  only 
on  account  of  their  distance  and  not  because  they 
are  intrinsically  small.  In  like  manner  the  remark- 
able fact  with  regard  to  the  nebulae  which  has 
been  disclosed  by  Keeler's  memorable  researches  with 
the  Crossley  Reflector  is  the  existence  not  alone  of 
the  great  nebulae,  but  of  unexpected  scores  of  thou- 
sands of  small  nebulae,  or  rather,  I  should  say,  of 
nebulae  which  appear  small,  though  doubtless  in  many 
cases  these  objects  are  intrinsically  quite  as  splendid 
as  the  Dumb-bell  Nebula  or  the  Nebula  in  Orion. 
They  only  seem  small  in  consequence  of  being  many 
times  further  from  us  than  are  the  more  famous 
objects. 

Professor  Keeler's  experience  was  a  remarkable 
one.  He  was  photographing  a  well-known  nebula 
with  the  Crossley  Reflector,  and  he  was  a  little  sur- 
prised to  find  that  on  the  same  plate  which  gave  him 
the  nebula  at  which  he  was  aiming  there  were  no 
fewer  than  seven  other  small  nebulous  objects  pre- 
viously unknown  to  astronomers.  It  at  first  appeared 
to  him  that  this  must  be  an  unusual  number  of 
nebulae  to  find  crowded  together  on  one  plate  which 
covered  no  more  than  one  square  degree  of  the  heavens, 
an  area  about  five  or  six  times  as  large  as  the  area 
of  the  full  moon.  Subsequent  experience,  however, 


DISCOVERING   NEW  NEBULA.  47 

showed  him  that  this  fact,  however  astonishing,  was 
not  at  all  unusual.  In  fact,  he  found  to  his  amaze- 
ment that,  expose  the  plate  where  he  pleased,  he 
generally  obtained  new  nebulae  upon  it,  and  sometimes 
even  a  much  larger  number  than  the  seven  which  so 
greatly  surprised  him  at  first.  I  may  mention  just  one 
or  two  instances.  There  is  a  well-known  and  interesting 
nebula  in  Pegasus  which  Professor  Keeler  photographed. 
When  he  developed  the  plate,  which,  of  course,  in- 
cluded a  considerable  region  of  the  heavens  in  the 
vicinity  of  the  particular  nebula,  he  found  to  his 
astonishment  that,  besides  the  nebula  he  wanted,  there 
were  not  less  than  twenty  other  nebulae  on  the  plate. 
But  there  is  a  more  striking  instance  even  than  this. 
A  plate  directed  to  a  part  of  the  constellation  of 
Andromeda,  with  the  object  of  taking  a  portrait  of  a 
particular  nebula  of  considerable  interest,  was  found 
to  contain  not  only  the  desired  nebula,  but  no  fewer 
than  thirty- one  other  new  nebulae  and  nebulous  stars. 
Nor  have  we  in  these  statements  exhausted  the  nebu- 
lous contents  of  these  wonderful  plates,  if  indeed 
we  have  rightly  interpreted  their  nature.  Professor 
Keeler  tells  us  that  he  finds  upon  them  a  considerable 
number  of  objects  which  in  all  probability  are  also 
nebulae,  though  they  are  so  small  that  the  telescope 
is  unable  to  reveal  them  in  their  true  character. 
Examination  does  little  more  than  show  these  objects 
as  points  of  light  which,  however,  are  apparently  not 
stars. 

In  the  remarkable  paper  from  which  I  have  taken 
these  facts  Professor  Keeler  makes  an  estimate  which 
is  founded  on  the  examination  of  his  plates.  If  the 
heavens  were  to  be  divided  into  panels,  each  one  square 


48  THE    EARTH'S    BEGINNING. 

degree  in  area,  there  would  be  about  forty  thousand 
panels.  It  follows  that  if  we  desired  to  photograph  the 
whole  heavens,  and  if  each  of  the  plates  was  to  cover 
one  square  degree,  forty  thousand  pictures  would  be 
needed  for  the  representation  of  the  whole  celestial 
sphere.  Keeler's  work  convinced  him  that  such  plates 
taken  by  the  Crossley  Reflector  would,  on  an  average, 
each  show  at  least  three  new  nebulae.  He  admitted 
it  is  quite  possible  that  there  may  be  regions  of  the 
sky  in  which  no  new  nebulae  are  to  be  found.  But 
in  the  regions  which  he  had  so  far  tested  he  in- 
variably found  more  than  three  nebulae  on  each  square 
degree ;  indeed,  as  we  have  seen,  on  some  of  his  plates 
he  found  a  much  larger  number  of  these  remark- 
able objects.  He  therefore  said  that  he  makes  but  a 
very  moderate  estimate  when  he  gives  a  hundred  and 
twenty  thousand  as  the  probable  number  of  the  new 
nebulae  within  the  reach  of  the  photographic  plates  of 
the  Crossley  Reflector. 

The  enormous  extension  which  these  investiga- 
tions have  given  to  our  knowledge  demands  the  serious 
attention  of  all  interested  in  the  heavens.  The  dis- 
coveries of  the  earlier  astronomers  had  led  to  the 
knowledge  of  about  six  thousand  nebulae  ;  the  Crossley 
Reflector  at  the  Lick  Observatory  has  now  rendered  it 
practically  certain  that  the  number  of  nebulae  in  the 
heavens  must  be  at  least  twenty-fold  as  great  as  had 
been  hitherto  supposed. 

In  subsequent  chapters  we  are  to  present  the  evi- 
dence for  the  belief  that  this  earth  of  ours,  as  well  as 
the  sun  and  all  the  other  bodies  which  form  the  solar 
system,  did  once  originate  in  a  nebula.  According  to 
this  view  the  materials  which  at  present  are  found  in 


Fig.  8.— THE  CROSSLEY  REFLECTOR  (CONSTRUCTED  BY  DR.  A.  A. 
COMMON    F.R.S.,  AND  NOW  AT  THE  LICK  OBSERVATORY). 


50  THE    EARTH'S    BEGINNING. 

the  globes  of  the  solar  system  were  once  distributed  over 
a  vast  extent  of  space  as  a  fire-mist,  or  nebula.  It  is 
surely  very  pertinent  to  be  able  to  show  that  a  nebula, 
such  as  we  suppose  to  have  been  the  origin  of  our 
system,  is  not  a  mere  figment  of  the  imagination.  No 
doubt  it  is  impossible  for  us  now  to  show  the  original 
nebula  from  which  the  solar  system  has  been  evolved. 
It  is  nevertheless  possible,  as  we  have  seen,  to  show  that 
a  hundred  and  twenty  thousand  nebulae  are  now  actually 
existing  of  every  grade  of  magnitude.  They  range  from 
such  magnificent  objects  as  the  Great  Nebula  in  Orion 
and  the  Dumb-bell  Nebula,  down  to  objects  wholly 
invisible,  not  merely  to  the  unaided  eye,  but  even  in 
the  most  powerful  telescope,  and  only  to  be  discerned 
as  hazy  spots  of  light  on  the  photographic  plates  of 
an  instrument  such  as  the  Crossley  Reflector. 

Though  no  eye  has  seen  the  actual  stages  in  the 
grand  evolution  of  our  solar  system,  we  may  at  least 
witness  parallel  stages  in  the  evolution  through  which 
some  of  the  myriads  of  other  nebulae  are  now  passing. 
We  find  some  of  these  nebulae  in  that  excessively 
diffused  condition  in  which  they  are  devoid  of  visible 
structure.  Material  in  this  form  may  be  regarded  as 
the  primaeval  nebula.  There  is  at  least  one  of  these 
extraordinary  objects  which  is  larger  a  great  deal  than 
even  the  Great  Nebula  in  Orion,  but  altogether  too  faint 
to  be  seen  except  by  the  photographic  plate.  Here  we 
find,  as  it  were,  the  mother  substance  in  its  most  ele- 
mentary stage  of  widest  possible  diffusion,  from  which 
worlds  and  systems,  it  may  be,  are  yet  to  be  evolved. 
From  diffused  objects  such  as  shown  in  Fig.  5  we  can 
pass  to  other  nebulae  in  which  we  see  a  certain  advance 
being  made  in  the.  process  by  which  the  nebula  is 


THE   NEBULAR    THEORY.  51 

transformed  from  the  primitive  condition;  and  we  can 
point  to  other  nebulae  in  which  the  advance  to  a  yet  fur- 
ther stage  of  development  is  more  and  more  pronounced. 
Thus  the  various  stages  in  the  evolution  of  a  system 
are  to  be  witnessed,  not  indeed  in  the  transformation  of 
a  single  nebula,  but  by  observing  a  properly  arranged 
series  of  nebulae  in  all  gradations,  from  the  diffused 
luminous  haze  to  a  star  with  a  faint  nebulous  surround- 
ing. Such  was  Herschel's  original  argument,  and  its 
cogency  has  steadily  increased  from  the  time  he  first 
stated  it  down  to  the  present  hour 


CHAPTER  IV. 

NEBULAE — APPARENT  AND  REAL. 

The  Globular  Star-clusters— Structure  of  these  Objects— Variability 
of  Stars  in  the  Cluster — Telescopic  Resemblance  of  a  Cluster  to 
a  Nebula — Resolution  of  a  Nebula — Supposition  that  all  Nebulae 
may  be  Clusters — A  Criterion  for  distinguishing  a  Nebula  and 
a  Cluster—  Dark  Lines  on  a  bright  Background  characterise  the 
Structure  of  a  Star — Bright  Lines  on  a  dark  Background  charac- 
terise the  Structure,  of  a  Nebula — Characteristics  of  the  Spectrum 
of  a  true  Nebula  and  of  a  Resolvable  Nebula — Spectra  of  the 
Sun  and  Capella — Spectra  of  the  Nebula  in  Orion  and  of  a  White 
Star  compared — Number  of  Lines  in  a  Nebular  Spectrum — Criterion 
of  a  Nebular  Spectrum — Spiral  Nebula  not  Gaseous — Solar  Spectra 
during  an  Eclipse — Bearing  on  the  Nebular  Theory — Herschel's 
Work — The  Objection  to  the  Theory— The  Objection  Removed 
in  1864. 

THERE  is  perhaps  hardly  any  telescopic  object  more 
pleasing  or  more  instructive  than  a  globular  cluster 
of  stars  when  viewed  through  an  instrument  suffi- 
ciently powerful  to  do  justice  to  the  spectacle.  There 
are  several  star-clusters  of  the  class  designated  as 
"  globular."  The  most  famous  of  these,  or,  at  all 
events,  the  one  best  known  to  northern  astronomers, 
is  found  in  the  constellation  of  Hercules,  and  is  for 
most  purposes  sufficiently  described  by  the  expression. 
"The  Cluster  in  Hercules."  The  genuine  lover  of 


THE    CLUSTER   IN   HERCULES. 


53 


Nature   finds  it   hard  to   withhold   an   exclamation  of 
wonder   and   admiration   when   for   the    first    time,   or 


Fig.  9.— THE  CLUSTER  IN  HERCULES. 

(Photographed  by  Dr.  W.  E.  Wilson,  F.R.S.) 

even  for  the  hundredth  time,  the  Cluster  in  Hercules 
is  adequately  displayed  in  the  field  of  a  first-class 
telescope. 


54  THE   EARTH'S   BEGINNING. 

In  Fig.  9  is  a  photograph  of  this  celebrated  object, 
which  was  taken  by  Dr.  W.  E.  Wilson,  F.K.S.,  at 
his  observatory  at  Daramona,  in  Ireland.  The  picture 
has  been  obtained  from  an  enlargement  of  the  original 
photograph  taken  with  the  telescope  in  Mr.  Wilson's 
observatory.  It  is,  however,  precisely  as  Nature  has 
given  it,  except  for  this  enlargement.  You  will  note 
that  towards  the  margin  of  the  cluster  the  several 
stars  are  seen  separately,  and  in  many  cases  with 
admirable  distinctness.  We  do,  however,  occasionally 
find  two  or  more  stars  so  close  together  that  their 
images  overlap;  and,  indeed,  in  the  centre  of  the 
cluster  the  stars  are  so  close  together  that  it  is  im- 
possible to  differentiate  them,  so  as  to  see  them  as 
individual  points  of  light.  We  need  have  no  doubt, 
however,  that  the  cluster  is  mainly  composed  of 
separate  stars,  although  the  difficulties  interposed  by 
our  atmosphere,  added  to  the  necessary  imperfections 
of  our  appliances,  make  it  impossible  for  us  to  dis- 
criminate the  individual  stars. 

In  looking  at  a  star  group  of  this  particular  kind 
the  observer  may  perhaps  be  reminded  of  a  swarm 
of  bees  in  flight  from  the  hive,  for  the  stars  in  the 
cluster  are,  on  a  vast  scale,  apparently  associated  in 
the  same  way  as  the  bees,  on  a  small  scale,  are  asso- 
ciated in  the  swarm.  We  may  also  compare  the  stars 
in  the  cluster  to  the  bees  in  the  swarm  in  another 
respect.  Each  bee  in  the  swarm  is  in  incessant 
movement.  There  can  be  no  doubt  that  each  star  in 
a  globular  cluster  is  unceasingly  changing  its  position 
with  reference  to  the  others.  The  distance  by  which 
the  cluster  is  separated  from  the  earth  renders  it  im- 
possible for  us  to  see  those  movements,  at  all  events 


HOW   STARS    VARY.  55 

within  those  narrow  limits  of  time  over  which  our 
observations  have  as  yet  extended;  but  the  laws  of 
mechanics  assure  us  that  the  mutual  attraction  of 
the  stars  in  this  cluster  must  give  rise  to  incessant 
movements,  and  that  this  must  be  the  case  notwith- 
standing the  fact  that  the  relative  places  of  the  stars 
in  the  cluster  show  no  alteration  that  can  be  recog- 
nised from  one  year's  end  to  another. 

I  may,  however,  mention  that  though  there  may 
be  no  movements  in  these  stars  great  enough  to  be 
observed,  yet  the  brightness  of  some  of  them  shows 
most  remarkable  fluctuations.  The  investigations  of 
Professor  Barnard  and  other  astronomers  have,  indeed, 
disclosed  such  curious  variability  in  the  brightness  of 
some  of  these  stars  that  if  it  were  not  for  the  ex- 
ceedingly high  authority  by  which  this  phenomenon 
has  been  guaranteed  we  should,  perhaps,  almost  hesi- 
tate to  believe  so  startling  a  fact.  It  has,  however, 
been  most  certainly  proved  that  many  of  the  stars 
in  certain  globular  clusters  pass  through  a  series 
of  periodical  changes  of  lustre.  The  period  is  a  very 
short  one  as  compared  with  the  periods  of  better 
known  variable  stars,  for  in  this  case  twenty-four 
hours  are  more  than  sufficient  for  a  complete  cycle 
of  changes,  and  it  not  infrequently  happens  that  in 
the  course  of  a.  single  quarter  of  an  hour  a  star  will 
lose  or  gain  brightness  to  the  extent  of  a  whole 
magnitude.  The  phenomenon  referred  to  is  at  the 
present  moment  engaging  the  careful  attention  of 
astronomers;  but  it  offers  a  problem  of  which,  indeed, 
it  is  not  at  present  easy  to  see  the  solution. 

Our  immediate  concern,  however,  with  the  globular 
star-clusters  relates  to  a  point  hardly  of  such  refine- 


56  THE    EARTH'S    BEGINNING. 

inent  as  that  to  which  I  have  just  referred ;  it  is  one 
of  a  much  more  elementary  nature.  The  photograph 
in  the  figure  may  be  considered  to  represent  the 
Cluster  in  Hercules  as  it  would  be  seen  with  a  tele- 
scope of  very  considerable  visual  power,  for  the  object 
would  assume  a  different  appearance  in  a  telescope 
which  was  not  first  class.  The  perfection  of  a  really 
powerful  instrument  is  tested  by  its  capability  of 
exhibiting  as  two  separate  points  a  pair  of  stars 
which  are  excessively  close  together,  and  which  in 
an  instrument  of  inferior  power  cannot  be  distin- 
guished, but  seem  fused  into  a  single  object.  The 
defining  power  of  a  telescope — that  is  to  say,  its 
capability  for  separating  close  double  stars — is  in- 
creased with  the  size  of  the  instrument,  always 
granting,  of  course,  that  there  is  equal  optical  perfec- 
tion in  both  cases.  It  follows  that  the  more  powerful 
the  telescope  the  more  numerous  are  the  stars  which 
can  be  seen  separately  in  a  globular  cluster. 

If,  however,  a  small  telescope  be  used,  or  a  tele- 
scope which,  though  of  considerable  size,  has  not  the 
high  optical  perfection  that  is  demanded  in  the  best 
modern  instruments,  then  adjacent  stars  are  not 
always  to  be  seen  separately.  It  may  be  that  the 
telescope,  on  account  of  its  small  size,  cannot  separate 
the  objects  sufficiently,  or  it  may  be  that  the  imper- 
fections of  the  telescope  do  not  present  the  star  as  a 
point  of  light,  but  rather  as  a  more  or  less  diffused, 
luminous  disc.  In  either  case  it  may  happen  that  a 
star  overlaps  other  stars  in  its  immediate  neighbour- 
hood, and  consequently  an  object  which  is  really  a 
cluster  of  separate  stars  may  fail  altogether  to  present 
the  appearance  of  a  cluster. 


HOW    TO    KNOW   A    NEBULA.  57 

I  have  been  alluding  to  something  which,  as  every 
astronomer  knows,  is  of  practical  importance  in  the 
observatory.  Like  every  one  else  who  has  ever 
used  a  telescope,  I  have  myself  seen  the  Cluster  of 
Hercules  with  just  the  same  misty  appearance  in  a 
small  telescope  that  an  undoubted  nebula  possesses 
in  the  very  finest  instrument.  It  is,  accordingly, 
sometimes  impossible,  merely  by  observation  with  a 
small  instrument,  to  distinguish  between  what  is 
certainly  a  cluster  of  stars  and  what  is  certainly  a 
nebula.  It  has  indeed  not  infrequently  happened 
that  an  observer  with  a  small  telescope  has  discovered 
what  appeared  to  him  to  be  a  nebula,  and  he  has 
recorded  it  as  such;  and  yet  when  the  same  object 
was  subsequently  examined  with  an  instrument  of 
greater  defining  power  the  nebulous  character  has 
been  seen  to  have  been  wrongly  attributed.  The 
object  in  such  a  case  is  proved  to  be  nothing  more 
than  a  cluster  of  stars,  of  which  the  individual 
members  are  either  intrinsically  faint  or  exceedingly 
remote;  it  certainly  is  not  a  mass  of  that  fire-mist 
or  gaseous  material  which  alone  is  entitled  to  be 
called  a  nebula. 

It  is  therefore  a  question  ot  importance  in  prac- 
tical astronomy  to  decide  whether  objects  which 
appear  to  be  nebulae  are  really  entitled  to  the  name, 
or  whether  the  nebulous  appearance  may  not  be  an 
optical  illusion.  The  operation  by  which  an  object 
previously  deemed  to  be  a  nebula  is  shown  by  the 
application  of  increased  telescopic  power  to  be  a 
cluster  of  stars  is  commonly  known  as  the  resolution 
of  a  nebula.  About  fifty  years  ago  the  mighty  six- 
foot  reflecting  telescope  of  Lord  Rosse,  and  other 


68  THE    EART&S    BEGINNING. 

great  instruments,  were  largely  employed  on  this 
work.  It  was,  indeed,  at  that  time  held  to  be  one 
of  the  special  tasks  which  came  most  legitimately 
within  the  province  of  the  big  telescopes,  to  show 
that  the  so-called  nebulae  of  earlier  observers  were 
resolvable  into  star- clusters  under  the  superior  powers 
now  brought  to  bear  upon  them. 

The  success  with  which  this  process  was  applied 
to  many  reputed  nebulae,  which  were  thereby  shown 
to  be  not  entitled  to  the  name,  led  not  unnaturally 
to  a  certain  conjecture.  It  was  admitted  that  certain 
objects  which  had  successfully  resisted  the  resolving 
powers  of  inferior  instruments  were  forced  to  con- 
fess themselves  as  mere  star-clusters  when  greatly 
increased  telescopic  power  was  brought  to  bear  on 
them;  and  it  was  conjectured  that  similar  success 
would  attend  the  attempts  to  resolve  still  other 
nebulae.  It  was  supposed  that  every  object  described 
as  a  nebula  could  only  be  entitled  to  bear  that 
designation  provisionally,  only  indeed  until  some  tele- 
scope of  sufficient  power  should  have  been  brought 
to  bear  on  it.  It  seemed  not  unreasonable  to  surmise 
that  every  one  of  the  so-called  nebulae  is  a  cluster 
of  stars,  even  though  a  telescope  sufficiently  powerful 
to  effect  its  resolution  might  never  be  actually  forth- 
coming. 

I  do  not,  indeed,  suppose  that  this  opinion  as  to  the 
ultimate  resolvability  of  all  nebulae  could  have  been 
shared  by  many  who  had  much  practical  experience  in 
the  actual  observation  of  these  objects  with  the  great 
telescopes,  for  the  particular  classes  of  nebulae  which  in 
telescopes  of  superior  powers  resolved  themselves  into 
groups  of  stars  had  a  characteristic  appearance.  After 


NEBULA  AND   STAR-CLUSTERS.  59 

a  little  experience  the  observer  soon  learned  to  recognise 
those  nebulae  which  promised  to  be  resolvable.  The 
object  might  not  indeed  be  resolvable  with  the  powers 
at  his  disposal,  but  yet  from  its  appearance  he  often  felt 
that  the  nebula  would  be  probably  resolved  if  ever  the 
time  should  come  that  greater  powers  were  applied  to 
the  task. 

It  is  easy  to  illustrate  the  question  at  issue  by  the 
help  of  the  photograph  of  the  Cluster  in  Hercules  in 
Fig.  9.  Each  of  the  stars  is  there  distinct,  except 
where  they  are  much  crowded  in  the  centre.  If,  how- 
ever, the  photograph  be  examined  through  one  of  those 
large  lenses  which  are  often  used  for  the  purpose,  and 
if  the  lens  be  held  very  much  out  of  focus,  the  stars 
will  not  be  distinguishable  separately,  and  the  whole 
object  will  be  merely  a  haze  of  light.  This  illustration 
may  help  to  explain  how  the  different  optical  conditions 
under  which  an  object  is  looked  at  may  exhibit,  at  one 
time  as  a  diffused  nebula,  an  object  which  in  better 
circumstances  is  seen  to  be  a  star-cluster. 

The  astronomer  who  was  fortunate  enough  to  have 
the  use  of  a  really  great  telescope  would  not  fail  to  notice 
that,  in  addition  to  the  so-called  nebulae  already  referred 
to,  which  were  presumably  resolvable,  there  were  certain 
other  objects,  generally  characterised  by  a  bluish  hue, 
which  in  no  circumstances  whatever  presented  the 
appearance  of  being  composed  of  separate  stars.  We 
now  know  for  certain  that  these  bluish  objects  are  not 
clusters  of  stars,  but  that  they  are  in  the  strictest 
sense  entitled  to  the  name  of  nebulae,  and  that  they  are 
gaseous  masses  or  mists  of  fire-cloud.  The  full  demon- 
stration of  this  important  point  was  not  effected  until 
1864. 


60  THE    EARTH1 8   BEGINNING. 

The  fact  that  so  very  many  of  the  nebulae  were  re- 
solved led  not  unreasonably  to  the  presumption  that  all 
the  nebulae  would  in  due  time  also  yield.  But  there  were 
many  who  could  not  accept  this  view,  and  there  was.  a 
long  discussion  on  the  subject.  At  last,  however,  the 
improvements  in  astronomical  methods  have  cleared  up 
the  question.  Sir  W.  Huggins  has  shown  that  there 
are  two  totally  distinct  classes  of  nebulae,  or  rather  of 
so-called  nebulae.  There  are  certain  nebulae  which  can 
be  resolved,  and  there  are  certain  nebulae  which  cannot. 
A  nebula  which  can  be  resolved  would  be  a  veritable 
cluster  of  stars,  and  is  not  really  entitled  to  the  name 
of  nebula  ;  a  nebula  which  cannot  be  resolved  would 
be  entitled  to  the  name,  for  it  is  a  volume  of  gas  or 
of  gaseous  material  which  is  itself  incandescent.  We 
have  been  provided  with  a  beautiful  criterion  by  which 
we  can  decide  to  which  of  these  classes  any  nebulous- 
looking  object  belongs. 

The  spectroscope  is  the  instrument  which  dis- 
criminates the  two  different  classes  of  objects.  This 
remarkable  apparatus,  to  which  we  owe  so  much  in 
every  department  of  astronomy,  receives  the  beam  of 
light  from  the  celestial  body.  The  instrument  then 
analyses  the  light  into  its  component  rays,  and  conducts 
each  one  of  those  rays  separately  to  a  distinct  place  on 
the  photographic  plate.  When  the  photograph  is 
developed  we  find  on  the  various  parts  of  the  plate  the 
evidence  as  to  the  class  of  rays  which  have  entered  into 
the  composition  of  the  light  that  has  been  submitted  to 
this  very  searching  form  of  examination. 

The  light  which  comes  from  a  star  or  any  star-like 
body,  including  the  sun  itself,  may  first  be  described. 
That  light,  after  passing  through  the  spectroscope  and 


SPECTRA    OF   STARS.  61 

having  been  conducted  to  the  photographic  plate,  wilj 
produce  a  picture  of  dark  lines  on  a  bright  background ; 
this  is,  at  least,  the  spectrum  which  a  star  generally 
presents.  There  are,  indeed,  many  types  of  stellar 
spectra,  for  there  are  many  different  kinds  of  stars,  and 
each  kind  of  star  is  conveniently  characterised  by  the 
particular  spectrum  that  it  yields.  If  the  star  be  one  of 
small  magnitude,  then  the  lines  in  its  spectrum  may  be 
detected,  but  only  with  great  difficulty.  It  not  infre- 
quently happens  that  the  photograph  of  the  spectrum  of 
such  a  star  will  show  no  more  than  a  continuous  band 
of  light  without  recognisable  lines ;  and  this  is  what 
occurs  in  the  case  of  a  resolvable  nebula,  where  the  stars 
are  so  closely  associated  that  the  spectrum  of  each 
separate  star  cannot  be  distinguished.  The  spectrum  of 
a  resolvable  nebula  is  merely  a  streak  of  light,  which  is 
the  joint  effect  of  all  the  spectra.  The  spectrum  is  then 
too  faint  to  show  the  rainbow  hues  which  present  such 
beautiful  features  in  the  spectrum  of  a  bright  star,  as 
they  do  in  the  spectrum  of  the  sun  itself. 

I  give,  in  the  adjoining  figure  (Fig.  10),  portions  of 
the  photographs  of  two  spectra  of  celestial  objects.  They 
have  been  taken  from  the  Atlas  of  representative  stellar 
spectra  in  which  Sir  William  and  Lady  Huggins  have 
recorded  the  results  of  their  great  labours.  Two  spectra 
are  represented  in  this  picture,  the  uppermost  being  the 
spectrum  of  the  sun,  while  the  lower  and  broader  one  is 
the  spectrum  of  the  bright  star  Capella.  It  has  not 
been  possible  within  the  limits  of  this  picture  to  include 
the  whole  length  of  these  two  spectra,  and  it  must  there- 
fore be  understood  that  the  photographs  given  in  the 
Atlas  are  each  about  five  times  as  long  as  the  parts 
which  are  here  reproduced. 


62  THE    EARTH'S    BEGINNING. 


Fig.  10. — Srx  AND  CAPELLA. 

Sun  above.  Capella  below. 

(Sir  William  and  Lady  Hugging.) 

But  the  characteristic  portions  of  the  spectra  selected 
are  sufficient  for  our  present  argument.  It  will  be  noted, 
first  of  all,  that  there  is  a  singular  resemblance  between 
the  details  of  the  spectrum  of  the  sun  and  those  of  the 
spectrum  of  the  star.  No  doubt  the  breadth  of  the 
stellar  picture  in  the  lower  line  is  greater  than  that  of 
the  solar  picture  in  the  upper  line ;  but  this  point  is  not 
significant.  The  breadth  of  the  spectrum  of  the  sun 
could  easily  have  been  made  as  wide  or  wider  if 
necessary.  The  breadth  is  immaterial,  for  the  character 
of  a  spectrum  is  determined  not  by  its  breadth,  but  by 
those  lines  which  cross  it  transversely.  It  will  be  seen 
that  there  are  here  a  multitude  of  lines,  some  being  very 
dark,  and  some  so  faint  as  to  be  hardly  visible.  Both 
spectra  exhibit  every  variety  of  line,  between  the  deli- 
cate marks  which  can  barely  be  seen  and  the  two 
bold  columns  on  the  right-hand  side  of  the  picture. 

The  characteristic  of  the  spectrum  is  given  by  the 
number,  the  arrangement,  the  breadth,  the  darkness, 
and  the  defmiteness  of  the  lines  by  which  it  is  crossed, 
and  the  first  point  that  we  note  is  the  remarkable 
resemblance  in  these  different  respects  between  the  two 


SPECTRAL    LIKENESSES.  63 

spectra.  The  lines  are  practically  identical,  at  least  so 
far  as  those  parts  of  the  spectrum  represented  in  this 
picture  are  concerned.  We  have  thus  a  striking  illus- 
tration of  the  important  fact,  to  which  we  have  so 
often  to  make  allusion,  of  the  general  resemblance 
of  the  sun  to  the  stars.  Not  only  do  we  know  that  if 
the  sun  were  removed  about  a  million  times  as  far  as  it 
is  at  present  its  light  would  be  reduced  to  that  of  a  star, 
but  that  the  star  Capella  transmits  to  us  light  con- 
sisting essentially  of  the  same  waves  as  those  which 
enter  into  a  beam  of  sunlight.  No  more  striking  illus- 
tration of  the  analogy  between  the  sun  and  a  star  can 
be  found  than  that  which  is  given  in  this  photograph 
from  the  famous  Observatory  at  Tulse  Hill. 

But  it  must  not  be  inferred  that  because  the  spectra 
of  sun  and  star  are  like  each  other,  they  are  therefore 
absolutely  identical.  There  are  many  lines  and  details 
to  be  seen  on  the  actual  photographic  plate  which  are 
too  delicate  to  be  reproduced  in  such  copies  as  it  is 
possible  to  make.  When  a  close  comparison  is  made 
on  the  actual  plate  itself  of  the  lines  in  the  solar  spec- 
trum and  the  lines  in  the  spectrum  of  Capella,  it  is 
observed  that,  though  they  are  the  same  so  far  as  the 
more  important  lines  are  concerned,  yet  that  there  are 
many  lines  found  in  the  spectrum  of  Capella  which 
are  not  found  in  the  spectrum  of  the  sun. 

The  contrast  between  the  spectrum  of  a  nebula 
properly  so  called  and  the  spectrum  of  a  star  is  well 
illustrated  by  the  accompanying  picture  (Fig.  11),  in 
which  Sir  W.  Huggins  exhibits  the  photograph  of  the 
spectrum  of  the  Nebula  in  Orion  in  comparison  with 
the  spectrum  of  a  star.  The  uppermost  of  the  two 
is  the  spectrum  of  the  star.  It  will  be  noted  that  this 


THE    EARTH'S    BEGINNING. 


Fig.  11. — SPECTRUM  OF  NEBULA  IN  ORION  AND  SPECTRUM  OF 
WHITE  STAR. 

(Sir  William  Huggins,  K.C.B.) 

spectrum  is  very  different  from  that  which  we  have 
already  seen  in  Capella.  Instead  of  a  vast  multitude  of 
lines  resembling  the  lines  of  the  solar  spectrum,  the  spec- 
trum of  a  star  of  the  type  here  represented,  of  which  we 
may  take  Sirius  as  the  most  striking  example,  exhibits 
but  a  few  lines.  We  regard  them  as  one  system  of 
lines,  for  we  know  they  are  physically  connected.  They 
are  all  alike  due  to  the  presence  of  a  single  element  in 
the  star,  that  element  being  in  fact  hydrogen.  But 
though  the  spectra  of  Capella  and  Sirius  are  so  totally 
different,  the  differences  relate  only  to  the  distribution 
of  the  lines,  and  to  their  number,  darkness,  and  width. 
In  both  cases  we  observe  the  characteristic  of  the 
light  from  an  ordinary  bright  star,  namely,  that  the 
spectrum  is  composed  of  a  bright  band  with  dark  lines 
across  it.  It  ought,  perhaps,  to  be  mentioned  here  that 
there  are  certain  very  special  stars  which  do  exhibit 
some  bright  lines  in  addition  to  a  more  ordinary 
spectrum;  this  is  especially  the  case  in  the  new  stars 
which  occasionally  appear.  Thus  in  the  case  of  the 
new  star  which  appeared  in  Perseus,  in  1901,  there 
were  several  remarkable  bright  lines.  This  most  inter- 
esting object  will  be  referred  to  again  in  a  later  chapter. 


THE    SPECTRUM   OF  A    NEBULA.  65 

Widely  different  from  the  spectrum  of  any  star 
whatever  is  the  lower  of  the  two  spectra  which  are 
shown  in  the  figure.  This  lower  spectrum  is  that  of 
the  Great  Nebula  in  Orion.  At  once  we  see  the 
fundamental  characteristic  of  a  nebula;  its  spectrum 
exhibits  five  bright  lines  on  a  dark  field.  I  do  not 
say  that  the  Great  Nebula  in  Orion  has  not  more 
than  five  lines ;  there  are  indeed  many  others,  for  Sir 
William  Huggins  has  himself  pointed  out  a  consider- 
able number,  and  the  labours  of  other  observers  have 
added  still  more  ;  but  the  five  lines  here  set  down  are 
the  principal  lines.  They  are  those  most  easily  seen; 
the  others  are  generally  extremely  delicate  objects 
arranged  in  groups  of  five  or  six.  But  the  lines  which 
this  picture  shows  are  quite  sufficient  to  exhibit  that 
fundamental  characteristic  of  the  nebular  spectrum, 
namely,  a  system  of  bright  lines  on  a  dark  field.  "  I 
may  further  mention  that  certain  lines  in  the  spectrum 
indicate  the  presence  of  the  element  hydrogen  in  the 
Great  Nebula  in  Orion,  and  we  owe  to  Dr.  Copeland 
the  interesting  discovery  that  the  remarkable  element 
helium  is  also  proved  to  exist  in  the  nebula. 

The  pictures,  at  which  we  have  been  looking,  will 
suffice  to  make  clear  the  criterion,  which  astronomers 
now  possess,  for  deciding  whether  an  object  which 
looks  nebulous  is  really  a  gaseous  nebula,  or  ought 
rather  to  be  regarded  as  a  star-cluster.  If  the  object 
be  a  star-cluster,  then  the  spectrum  that  it  gives  will 
be  the  resultant  of  the  spectra  of  the  stars,  and  this 
will  be  a  continuous  band  of  light.  If  the  stars  are 
bright  enough,  it  may  be  that  dark  lines  can  be 
detected  crossing  the  spectra,  but  in  the  case  of  the 
clusters  it  will  be  more  usual  to  find  the  continuous 


66  THE    EARTH'S    BEGINNING. 

band  of  light  so  faint  that  the  dark  lines,  even  if  they 
are  there,  are  not  distinguishable. 

If,  on  the  other  hand,  the  object  at  which  we  are 
looking,  not  being  a  cluster  of  stars,  is  indeed  a  mass 
of  glowing  gas,  or  true  nebula,  then  the  spectrum  that 
it  sends  us  is  not  the  continuous  spectrum  such  as 
we  expect  from  the  stars.  The  spectrum  which  the 
nebula  proper  transmits  to  the  plate  is  said  to  be 
discontinuous.  In  some  cases  it  is  characterised  by 
only  a  single  bright  line,  and  in  others  there  may  be 
two,  or  three,  or  four  bright  lines,  or,  as  in  the  case 
shown  in  Fig.  11,  the  number  of  bright  lines  may  be 
as  many  as  five.  It  may  indeed  happen,  in  the  case 
of  some  exquisite  photographs,  that  the  number  of 
lines  in  the  spectrum  of  the  nebula  will  be  increased 
to  a  score  or  possibly  more.  There  may  also  be  faint 
traces  of  a  continuous  spectrum  present,  this  being 
due  to  the  stars  scattered  through  the  object,  from 
which  perhaps  even  the  most  gaseous  nebula  is  not 
entirely  free.  But  the  characteristic  type  of  nebular 
spectrum  is  that  in  which  the  bright  lines,  be  they 
one,  or  few,  or  many,  are  separated  by  intervals  of  per- 
fect darkness.  When  it  is  found  that  the  spectrum 
of  a  nebula  can  be  thus  described,  it  is  correct  to 
say  that  the  nebula  is  truly  a  gaseous  object. 

In  the  lists  given  by  Scheiner  in  his  interesting 
book,  "Astronomical  Photography,"  the  number  of 
gaseous  nebulae  is  set  down  as  seventy-three.  Of 
course  no  one  pretends  that  this  enumeration  is  ex- 
haustive. It  claims  to  be  no  more  than  a  statement 
of  the  number  of  nebulae  which  have  been  proved,  by 
observations  made  up  to  the  present,  to  be  of  a  gaseous 
description.  Seeing  that  there  are,  as  we  have  already 


SPIRAL   NEBULA.  67 

stated,  many  scores  of  thousands  of  nebulous-looking 
objects,  it  is  highly  probable  that  the  number  above 
given  is  only  a  mere  fraction  of  the  number  of  gaseous 
nebulae  actually  within  reach  of  our  instruments. 

It  may,  however,  be  assumed  that  more  than  half 
the  objects  which  are  called  nebulae  are  not  of  the 
gaseous  type.  This  is  a  point  of  some  importance, 
which  appears  to  follow  from  the  facts  stated  by 
Professor  Keeler  in  connection  with  his  memorable  re- 
searches with  the  Crossley  Reflector.  In  a  later  chapter 
we  discuss  important  questions  connected  with  what 
are  called  spiral  nebulae.  We  may,  however,  here  record 
that  no  spiral  nebulae  have  as  yet  been  pronounced 
gaseous.  Professor  Keeler  assures  us  that,  of  the  one 
hundred  and  twenty  thousand  nebulae  which  he  esti- 
mates to  be  within  reach  of  the  Crossley  Reflector,  far 
more  than  half  are  of  the  spiral  character.  If,  then, 
we  assume  that  the  spectra  of  spiral  nebulae  are  always 
continuous,  it  seems  to  follow  that  less  than  half  the 
nebulous  contents  of  the  heavens  possesses  the  discon- 
tinuous spectrum  which  is  characteristic  of  a  gaseous 
object. 

We  are  not  entitled  to  assume  that  a  nebula,  or 
reputed  nebula,  which  shows  a  continuous  spectrum, 
must  necessarily  be  a  cluster,  not  merely  of  star-like 
bodies,  but  of  bodies  with  masses  comparable  with 
those  of  the  ordinary  stars.  Our  argument  does 
most  certainly  suggest  that  the  body  which  yields  a 
continuous  spectrum  is  not  a  gaseous  body ;  but  it 
may  be  g°ing  too  far  to  assert  that  therefore  it  is  a 
cluster  of  stars  in  the  ordinary  sense.  We  do  often 
find  true  nebulae  and  star-clusters  in  close  association. 
The  Nebula  in  the  Pleiades  (Fig.  13)  is  an  example. 


68  THE    EARTH'S    BEGINNING. 

It  may  be  desirable  to  add  a  few  words  here  as 
to  the  physical  difference  between  a  continuous  spec- 
trum and  a  discontinuous  spectrum.  If  the  light  from 
a  body,  known  to  be  gaseous,  be  examined,  it  shows 
the  discontinuous  spectrum  of  bright  lines  upon  a  dark 
background.  If,  on  the  other  hand,  a  solid  be  raised 
to  incandescence,  such,  for  instance,  as  a  platinum  wire 
heated  white-hot  by  an  electric  current,  or  a  cylinder 
of  lime  submitted  to  an  oxyhydrogen  blowpipe,  then 
the  spectrum  that  it  yields  is  continuous.  All  the 
colours  of  the  rainbow,  red,  orange,  yellow,  green,  blue, 
indigo,  violet,  are  shown  in  such  a  spectrum  as  a  con- 
tinuous band  of  light,  though  the  band  is  not  crossed 
by  dark  lines.  It  would  therefore  appear  that  the  con- 
tinuous spectrum  is  characteristic  of  an  incandescent 
solid,  and  the  discontinuous  spectrum  of  a  glowing 
gas.  But  here  it  may  be  urged  that  the  sun  presents 
a  difficulty.  We  so  often  refer  to  the  spectrum  of 
the  sun  as  continuous,  that  it  might  at  first  appear 
as  if  the  spectrum  of  the  sun  resembled  that  pro- 
duced by  radiation  from  a  solid  body.  But,  as  is 
well  known,  the  sun  is  not  a  solid  body.  Even  if 
the  sun  be  solid  at  the  centre,  it  is  certainly  far  from 
being  solid  in  those  superficial  regions  called  the 
photosphere,  from  which  alone  its  copious  radiation  is 
emitted.  If  the  sun  is  not  a  solid  body,  how  comes  it 
to  emit  a  radiation  characterised  in  the  same  way  as 
the  radiation  from  a  white-hot  solid  ?  Why  does  the 
solar  spectrum  not  exhibit  features  characteristic  of 
radiation  from  an  incandescent  gas  ?  The  point  is 
well  worthy  of  attention;  it  finds  an  explanation  in 
the  nature  of  the  photosphere  from  which  the  sun's 
radiation  proceeds. 


THE    SPECTEUM   OF    THE    SUN.  69 

The  photosphere,  though  not,  of  course,  to  be  de- 
scribed as  a  solid  body,  does  not  most  certainly,  so  far 
as  its  radiation  is  concerned,  behave  like  a  gaseous 
body.  In  the  glowing  clouds  of  the  photosphere  the 
carbon,  of  which  they  are  composed,  is  not  in  the 
gaseous  form ;  it  has  passed  into  solid  particles,  and 


Fig.  12. — SOLAR  SPECTRA  WITH  BRIGHT   LINES  AND  DARK 
LINES  DURING  ECLIPSE. 

(Photographed  by  Captain  Hills,  R.E.) 

it  is  these  particles,  in  the  highest  condition  of  incan- 
descence, which  emit  the  solar  radiation.  Although 
these  particles  are  sustained  by  the  gases  of  the  sun, 
and  are  associated  in  aggregations  which  form  the 
dazzling  clouds  of  the  photosphere,  yet  each  one  of 
them,  in  so  far  as  its  individual  radiation  is  concerned, 
ought  to  be  regarded  as  a  solid  body.  The  radiation 
from  the  sun  is,  therefore,  essentially  not  the  radiation 
from  an  incandescent  gas;  it  is  the  radiation  from  a 
glowing  solid.  This  is  the  reason  why  the  solar  spec- 
trum is  of  the  continuous  type. 

By  the  kindness   of  Captain  Hills,  R.E.,  I  am   able 

to  show  a  photograph  (Fig.  12)  containing  two  spectra 

taken  during  a  recent  eclipse,  which  will   serve  as   an 

excellent  illustration  of  the  different  points  which  we 

6 


70  THE    EARTH'S    BEGINNING. 

have  been  discussing.  It  is,  indeed,  true  that  neither 
of  the  spectra,  here  referred  to,  belongs  to  nebulae, 
whether  genuine  gaseous  objects  or  not.  Both  of  the 
spectra  in  Captain  Hills'  picture  are  actually  taken 
from  the  sun.  The  conditions  under  which  these 
spectra  were  obtained  makes  them,  however,  serve  as. 
excellent  illustrations  of  the  different  types  of  spectra. 
We  are  to  notice  that  the  upper  band,  which  contains 
what  is  called  the  "flash"  spectrum,  exhibits  bright 
lines  on  a  dark  background.  See,  for  instance,  the  two 
lines  so  very  distinctly  marked,  which  are  indicated 
by  the  letters  H  and  K.  These  lines  are  very  charac- 
teristic of  the  solar  spectrum,  and  it  may  be  mentioned 
that  they  are  indications  of  the  presence  of  a  well- 
known  element.  These  lines  prove  that  the  sun  con- 
tains calcium,  the  metal  of  which  common  lime  is  the 
oxide.  It  is,  indeed,  the  presence  of  this  substance  in 
the  sun  which  gives  rise  to  these  lines.  We  shall 
refer  again  to  this  subject  in  a  later  chapter. 

As  the  upper  of  the  two  spectra  exhibit  H  and  K 
as  white  lines  on  a  dark  background,  so  the  lower 
represents  the  same  lines  as  dark  objects  on  a  white 
background.  These  photographs  give  illustrations  of 
spectra  of  the  two  different  classes  which  provide 
means  of  discriminating  between  a  genuine  nebula 
and  an  object  which,  though  it  looks  like  a  nebula,  is 
not  itself  gaseous. 

But,  it  will  be  asked,  how  can  the  spectra  of  the 
two  distinct  types  both  be  obtained  from  the  sun  ?  The 
explanation  of  this  point  is  an  interesting  one.  The 
lower  of  the  two  is  the  ordinary  solar  spectrum ;  it  is 
a  continuous  spectrum  showing  dark  lines  on  a  bright 
field.  The  upper  spectrum,  which  shows  bright  lines 


Fig.  13. — THE  NEBULA  IN  THE  PLEIADES  (Exposure  10  hours). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 


72  THE    EARTHS    BEGINNING. 

on  a  dark  field,  is  produced  by  a  small  part  of  the 
sun  just  at  the  moment  when  the  eclipse  is  total 
The  circumstances  in  which  that  picture  was  secured 
will  explain  its  character.  The  moon  had  com- 
pletely covered  that  dazzling  part  of  the  sun  which 
we  ordinarily  see,  but  a  region  of  intensely  glowing 
gaseous  material  in  the  sun's  atmosphere  was  too  high 
above  the  surface  to  be  completely  hidden  by  the 
moon.  The  spectrum  of  this  region,  consisting  of  the 
characteristic  bright  gaseous  lines,  is  here  represented." 
The  ordinary  light  of  the  sun  being  cut  off,  opportunity 
was  thus  afforded  for  the  production  of  the  spectrum 
of  the  light  from  the  glowing  gas,  and  we  see  this 
spectrum  to  be  of  the  nebular  type. 

And  now  we  may  bring  this  chapter  to  a  close  by 
calling  attention  to  the  very  important  bearing  which 
its  facts  have  on  the  Nebular  Theory.  It  is  essential 
for  us  to  see  how  far  modern  investigation  and  dis- 
covery have  tended  either  to  substantiate  or  refute 
that  famous  doctrine  which  traces  the  development  of 
the  solar  system  from  a  nebula.  To  do  this  it  is 
necessary  to  contrast  the  knowledge  of  nebulae,  as  it- 
exists  at  present,  with  the  knowledge  of  nebulae  as  it 
existed  in  the  days  of  Kant  and  Laplace  and  Herschel. 

We  assuredly  do  no  injustice  to  Kant  or  to  La- 
place if  we  say  that  their  actual  knowledge  of  the 
nebulous  contents  of  the  heavens  was  vastly  inferior 
to  that  possessed  by  Herschel.  There  is  not  a  single 
astronomical  observation  of  nebulae  recorded  by  either 
Kant  or  Laplace ;  it  may  be  doubted  whether  either  of 
them  ever  even  saw  a  nebula.  Their  splendid  contri- 
butions to  science  were  made  in  directions  far  removed 
from  those  of  the  practical  observer,  who  passes  long 


HERSCHEVS    DISCOVERIES.  73 

hours  of  darkness  in  the  scrutiny  of  the  celestial 
bodies.  Herschel,  on  the  other  hand,  was  pre-emi- 
nently an  observer.  His  nights  were  spent  in  the 
most  diligent  practical  observation  of  the  heavens, 
and  at  all  times  the  nebulae  were  the  objects  which 
received  the  largest  measure  of  his  attention,  with  the 
result  that  the  knowledge  of  nebulae  received  the  most 
extraordinary  development  from  his  labours.  Earlier 
astronomers  had  no  doubt  observed  nebulae  occasionally, 
but  with  their  imperfect  appliances  only  the  brighter 
of  these  objects  were  discernible  by  them.  The  as- 
tonishing advance  made  by  the  observations  of  Herschel 
is  only  paralleled  by  the  advance  made  a  hundred  years 
later  by  the  photographs  of  Keeler. 

But  it  must  be  remembered  that  though  .Herschel 
observed  nebulae,  and  discovered  nebulae,  and  dis- 
coursed on  nebulae  in  papers  which  to  this  day  are 
classics  in  this  important  subject,  yet  not  to  the  last 
day  of  his  life  could  he  have  felt  sure  that  he  had 
ever  seen  a  genuine  nebula.  He  might  have  surmised, 
and  he  did  surmise,  that  many  of  the  objects  he  set 
down  as  nebulae  were  actually  gaseous  objects,  but  he 
knew  that  many  apparent  nebulae  were  in  truth  clus- 
jters  of  stars,  and  he  had  no  means  of  knowing  whether 
all  so-called  nebulae  might  not  belong  to  the  same 
category. 

It  was  not  till  nearly  half  a  century  after  Sir 
William  Herschel's  unrivalled  career  had  closed  that 
the  spectroscope  was  invoked  to  decide  finally  on  the 
nature  of  these  mysterious  objects.  That  decision, 
which  has  been  of  such  transcendent  importance  in 
the  study  of  the  heavens,  was  not  pronounced  till 
1864.  In  that  year  Sir  William  Huggins  established 


OF  THE 

UNIVERSITY 


74  THE   EARTH'S    BEGINNING. 

the  fundamental  truth  that  the  so-called  nebulae  are 
not  all  star-clusters,  but  that  the  universe  does  contain 
objects  which  are  most  certainly  gigantic  volumes  of 
incandescent  gases. 

This  great  achievement  provided  a  complete  answer 
to  those  who  urged  an  objection,  which  seemed  once 
very  weighty,  against  the  Nebular  Theory.  It  must  be 
admitted  that  before  1864  no  one  could  have  affirmed 
with  confidence  that  any  genuine  nebula  really  existed. 
It  was,  therefore,  impossible  for  the  authors  of  the 
Nebular  Theory  to  point  to  any  object  in  the  heavens 
which  might  have  illustrated  the  great  principles  in- 
volved in  the  theory.  The  Nebular  Theory  required 
that  in  the  beginning  there  should  have  been  a 
gaseous  nebula  from  which  the  solar  system  has  been 
evolved.  But  the  objector,  who  was  pleased  to  con- 
tend that  the  gaseous  nebula  was  a  figment  of  the 
imagination,  could  never  have  been  effectively  silenced 
by  Kant  or  Laplace  or  Herschel.  It  would  have  been 
useless  for  them  to  point  to  the  Nebula  in  Orion,  for 
the  objector  might  say  that  it  was  only  a  cluster  of 
.stars,  and  at  that  time  there  would  have  been  no  way 
of  confuting  him. 

The  authors  of  the  Nebular  Theory  had,  in  respect 
to  this  class  of  objector,  a  much  more  difficult  task 
than  falls  to  its  modern  advocate.  The  latter  is  able 
to  deny  in  the  most  emphatic  manner  that  a  gaseous 
nebula  is  no  more  than  an  imaginary  conception.  He 
can  now  demonstrate  that  the  Great  Nebula  in  Orion 
and  the  Dumb-bell  Nebula,  to  mention  only  two,  are 
assuredly  gaseous. 

The  famous  discovery  of  Sir  W.  Huggins  has  re- 
moved the  first  great  objection  to  the  Nebular  Theory. 


CHAPTER  V. 

THE   HEAT   OF  THE   SUN. 

The  Sun  to  be  first  considered  :  its  Evolution  is  in  vigorous  Progress 
— Considerations  on  Solar  Heat — Size  of  the  Sun — Waste  of  Sun- 
heat — Langley's  Illustration — Sun  in  Ancient  Days — Problem 
Stated— The  Solar  Constant  explained — Its  Value  determined — 
Estimate  of  Radiation  from  a  Square  Foot  of  the  Sun — Illustra- 
tions of  Solar  Energy — Decline  of  Solar  Energy — The  Warehouse 
of  Grain — White-hot  Globe  of  Iron  would  Cool  in  Forty-eight 
Years — Sun's  Heat  is  not  sustained  by  Combustion — Inadequacy 
of  Combustion  Demonstrated — Joule's  Unit — Energy  of  a  Moving 
Body — Energy  of  a  Body  moving  Five  Miles  a  Second — Energy 
of  the  Earth  due  to  its  Motion. 

IT  will  be  convenient  to  consider  different  bodies  in 
the  solar  system,  and  to  study  them  with  the  special 
object  of  ascertaining  what  information  they  aiford  as 
to  the  great  celestial  evolution.  We  cannot  hesitate 
as  to  which  of  the  bodies  should  first  claim  our 
attention.  Not  on  account  of  the  predominant  im- 
portance of  our  sun  to  the  inhabitants  of  the  earth, 
but  rather  because  the  sun  is  nearly  a  thousand 
times  greater  than  the  greatest  of  the  planets,  do  we 
assign  to  the  great  luminary  the  first  position  in  this 
discussion. 

The   sun  is,   indeed,   especially   instructive   on    the 


76  THE   EARTH'S   BEGINNING. 

subject  with  which  we  are  occupied.  By  reason  of 
its  great  mass,  the  process  of  evolution  takes  place 
more  slowly  in  the  sun  than  in  the  earth  or  in  any 
other  planet.  Evolution  has,  no  doubt,  largely  trans- 
formed the  sun  from  its  primaeval  condition,  but  it  has 
not  yet  produced  a  transformation  so  radical  as  that 
which  the  earth  and  the  other  planets  have  under- 
gone. On  this  account  the  sun  can  give  us  informa- 
tion about  the  process  of  evolution  which  is  not  to 
be  so  easily  obtained  from  any  of  the  other  heavenly 
bodies.  The  sun  can  still  exhibit  to  us  some  vestiges, 
if  we  may  so  speak,  of  that  great  primaeval  nebula 
from  which  the  whole  system  has  sprung. 

The  heat  of  the  sun  is  indeed  one  of  the  most 
astonishing  conceptions  which  the  study  of  Nature 
offers  to  us.  Let  me  try  to  illustrate  it.  Think  first 
of  a  perfect  modern  furnace  in  which  even  steel 
itself,  having  first  attained  a  dazzling  brilliance,  can 
be  further  melted  into  a  liquid  that  will  run  like 
water.  Let  us  imagine  the  temperature  of  that  liquid 
to  be  multiplied  seven-fold,  and  then  we  shall  obtain 
some  conception  of  the  fearful  intensity  of  the  heat 
which  would  be  found  in  that  wonderful  celestial 
furnace  the  great  sun  in  the  heavens. 

Ponder  also  upon  the  stupendous  size  of  that  orb, 
which  glows  at  every  point  of  its  surface  with  the 
astonishing  fervour  that  this  illustration  suggests. 
The  earth  on  which  we  stand  is  a  mighty  globe; 
yet  what  are  the  dimensions  of  our  earth  in  com- 
parison with  those  of  the  sun  ?  If  we  represent  the 
earth  by  a  grain  of  mustard  seed,  then  on  the  same 
scale  the  sun  should  be  represented  by  a  cocoanut. 
We  may  perhaps  obtain  a  more  impressive  concep- 


THE    SUN'S    HEAT.  77 

tion  of  the  proportions  of  the  orb  of  day  in  the 
following  manner.  Look  up  at  the  moon  which  re- 
volves round  the  heaven,  describing  as  it  does  so 
majestic  a  track  that  it  is  generally  at  a  distance  of 
two  hundred  and  forty  thousand  miles  from  the 
earth.  Yet  the  sun  is  so  large  that  if  there  were  a 
hollow  globe  equally  great,  and  the  earth  were  placed 
at  its  centre,  the  entire  orbit  of  the  moon  would 
lie  completely  within  it. 

Every  portion  of  that  stupendous  desert  of  flame 
is  pouring  forth  torrents  of  heat.  It  has,  indeed,  been 
estimated  that  the  heat  which  issues  from  an  area  of 
two  square  feet  on  the  sun  would  more  than  suffice,  if  it 
could  be  all  utilised,  to  drive  the  engines  of  the  largest 
Atlantic  liner  between  Liverpool  and  New  York. 

This  solar  heat  is  scattered  through  space  with 
boundless  prodigality.  No  doubt  the  dwellers  on  the 
earth  do  receive  a  fair  supply  of  sunbeams ;  but  what  is 
available  for  the  use  of  mankind  can  be  hardly  more 
than  an  infinitesimal  fraction  of  what  the  sun  emits. 
We  shall  scarcely  be  so  presumptuous  as  to  suppose  that 
the  sun  has  been  designed  solely  for  the  benefit  of  the 
poor  humanity  which  needs  light  and  warmth.  The 
heat  and  light  daily  lavished  by  the  sun  would  suffice 
to  warm  and  to  illuminate  two  thousand  million  globes, 
each  as  great  as  the  earth.  If,  indeed,  it  were  true 
that  the  only  object  of  the  sun's  existence  was  to 
cherish  this  immediate  world  of  ours,  then  all  we  can 
say  is  that  the  sun  carries  on  its  business  in  a  most 
outrageously  wasteful  manner.  What  would  be  thought 
of  the  prudence  of  one  who,  having  been  endowed  with 
a  fortune  of  ten  million  pounds,  spent  one  single  penny 
of  that  vast  sum  in  a  profitable  manner  and  dissipated 


78  THE    EARTH'S    BEGINNING. 

every  other  penny  and  every  other  pound  of  his  fortune 
in  aimless  extravagance  ?  But  this  is  apparently  the 
way  in  which  the  sun  manages  its  affairs,  so  far  as  our 
earth  is  concerned.  Out  of  every  ten  million  pounds 
worth  of  heat  issuing  from  the  glorious  orb  of  day, 
wlT  on  this  earth  secure  one  pennyworth,  and  all  but 
that  solitary  pennyworth  seems  to  be  utterly  squan- 
dered. We  may  say  it  certainly  is  squandered  so  far 
as  humanity  is  concerned.  What,  indeed,  its  actual 
destination  may  be  science  is  unable  to  tell. 

And  now  for  the  great  question  as  to  how  the  sun's 
heat  is  sustained.  How  is  it  that  this  career  of  tremen- 
dous prodigality  has  not  ages  ago  been  checked  by 
absolute  exhaustion  ?  Every  child  knows  that  the  fire 
on  the  hearth  will  go  out  unless  coal  be  provided. 
The  workman  knows  that  his  devouring  furnace  in  the 
ironworks  requires  to  be  incessantly  stoked  with  fresh 
supplies  of  fuel.  How,  then,  comes  it  that  the  won- 
derful furnace  on  high  can  still  continue,  as  it  has  con- 
tinued for  ages,  to  pour  forth  its  amazing  stores  of  heat 
without  being  exhausted  ? 

Professor  Langley  has  supplied  us  with  an  admirable 
illustration  showing  the  amount  of  fuel  which  would 
be  necessary,  if  indeed  it  were  by  successive  additions 
of  fuel  that  the  sun's  heat  was  sustained.  Suppose  that 
all  the  coal-seams  which  underlie  England  and  Scot- 
land were  made  to  yield  up  their  stores;  that  the 
vast  coalfields  in  America,  Australia,  China,  and  else- 
where were  compelled  to  contribute  every  combustible 
particle  they  contained;  suppose,  in  fact,  that  we  ex- 
tracted from  this  earth  every  ton  of  coal  which  it 
possesses  in  every  isle  and  every  continent;  suppose 
that  this  mighty  store  of  fuel,  sufficient  to  supply  all 


THE    SUN'S    AGE.  79 

the  wants  of  the  earth  for  centuries,  were  to  be  accumu- 
lated, and  that  by  some  mighty  effort  that  mass  were  to  be 
hurled  into  the  sun  and  were  -forthwith  to  be  burnt  to 
ashes ;  there  can  be  no  doubt  that  a  stupendous  quantity 
of  heat  would  be  produced.  But  what  is  that  heat  in 
comparison,  we  do  not  say  with  the  heat  of  the  sun, 
but  with  the  daily  expenditure  of  the  sun's  heat  ?  How 
long,  think  you,  would  the  combustion  of  so  vast  a  mass 
of  fuel  provide  for  the  sun's  expenditure  ?  We  are 
giving  deliberate  expression  to  a  scientific  fact  when  we 
say  that  a  conflagration  which  destroyed  every  particle 
of  coal  contained  in  this  earth  would  not  generate  as 
much  heat  as  the  sun  lavishes  in  the  tenth  part  of  every 
single  second.  During  the  few  minutes  that  you  have 
been  reading  these  words  a  quantity  of  heat  has  gone 
for  ever  from  the  sun  which  is  five  thousand  times  as 
great  as  all  the  heat  that  ever  has  been  or  ever  will  be 
produced  by  the  combustion  of  the  coal  that  this  earth 
has  furnished. 

But  we  have  still  another  conception  to  introduce 
before  we  can  appreciate  the  full  significance  of  the 
sun's  extraordinary  expenditure  of  heat  and  light.  We 
have  been  thinking  of  the  sun  as  it  shines  now  ;  but  as 
the  sun  shines  to-day,  so  it  has  shone  yesterday,  and 
so  it  shone  a  hundred  years  ago,  a  thousand  years  ago ; 
so  it  shone  in  the  earliest  dawn  of  history,  so  it  shone 
during  those  still  remoter  periods  when  great  animals 
flourished  which  have  now  vanished  for  ever ;  so  the  sun 
shone  during  those  remote  ages  when  life  began  to  dawn 
on  an  earth  which  still  was  young.  We  do  not,  indeed, 
say  that  the  intensity  of  the  sunbeams  has  remained 
actually  uniform  throughout  a  period  so  vast  ;  but 
there  is  every  reason  to  believe  that  throughout  these 


80  TEE    EARTH'S    BEGINNING. 

illimitable  periods  the   sun   has  expended  its  radiance 
with  the  most  lavish  generosity. 

A  most  important  question  is  suggested  by  these 
considerations.  The  consequences  of  frightful  ex- 
travagance are  known  to  us  all;  we  know  that  such 
conduct  tends  to  bankruptcy  and  ruin;  and  certainly 
the  expenditure  of  heat  by  the  sun  is  the  most  mag- 
nificent extravagance  of  which  our  knowledge  gives  us 
any  conception.  Accordingly,  the  important  question 
arises :  As  to  how  the  consequences  of  such  awful  pro- 
digality have  been  hitherto  averted.  How  is  it  that 
the  sun  is  still  able  to  draw  on  its  heat  reserve,  from 
year  to  year,  from  century  to  century,  from  seon  to 
aeon,  ever  squandering  two  thousand  million  times  as 
much  heat  as  that  which  genially  warms  our  tempe- 
rate regions,  as  that  which  draws  forth  the  exuberant 
vegetation  of  the  tropics  or  which  rages  in  the  desert 
of  Sahara  ?  That  is  the  great  problem  to  which  our 
attention  has  to  be  given. 

We  must  first  ascertain,  with  such  precision  as  the 
circumstances  permit,  the  actual  amount  of  heat  which 
the  sun  pours  forth  in,  its  daily  radiation.  The  deter- 
mination of  this  quantity  has  engaged  the  attention 
of  many  investigators,  and  the  interpretation  of  their 
results  is  by  no  means  free  from  a  difficulty.  It  is  to  be 
observed  that  what  we  are  now  seeking  to  ascertain  is 
not  exactly  a  question  of  temperature,  but  of  something 
quite  different.  What  we  have  to  measure  is  a  quantity 
of  heat,  which  is  to  be  expressed  in  the  proper  units  for 
quantities  of  heat.  The  unit  of  heat  which  we  shall 
employ  is  the  quantity  of  heat  necessary  to  raise  one 
pound  of  water  through  one  degree  Fahrenheit. 

The  solar  constant  is  the  number  of  units  of  heat 


THE    SOLAR    CONSTANT. 


Fig.  14.— THE  SUN  (July  8th,  1892). 

(Royal  Observatory,  Greenwich.) 
{From  the  Royal  Astronomical  Society  Series.) 

which  fall,  in  one  minute,  on  one  square  loot  ot  a 
surface  placed  at  right  angles  to  the  sun's  rays,  and 
situated  at  the  mean  distance  of  the  earth  from  the  sun. 
We  shall  suppose  that  losses  due  to  atmospheric  absorp- 
tion have  been  allowed  for,  so  that  the  result  will 
express  the  number  of  units  of  heat  that  would  be 
received  in  one  minute  on  a  square  foot  turned  directly 
to  the  sun,  and  at  a  distance  of  93,000,000  miles. 

This  is  a  matter  for  determination  by  actual  obser- 
vation and  measurement.  Theory  can  do  little  more 
than  suggest  the  precautions  to  be  observed  and  discuss 


82  THE   EARTH'S   BEGINNING. 

the  actual  figures  which  are  obtained.  There  have  been 
many  different  methods  of  making  the  observations,  and 
the  results  are  somewhat  various,  but  the  discrepancies 
are  not  greater  than  might  be  expected  in  an  investiga- 
tion of  such  difficulty.  The  mean  value  which  has  been 
arrived  at  is  fourteen,  and  the  fundamental  fact  with 
regard  to  the  solar  radiation  which  we  are  thus  enabled 
to  state  is  that  an  area  of  a  square  foot  exposed  at 
right  angles  to  the  solar  rays,  at  a  distance  of  93  mil- 
lions of  miles,  will  in  each  minute  receive  from  the 
sun  as  much  heat  as  would  raise  one  pound  of  water 
fourteen  degrees  Fahrenheit. 

It  follows  that  the  total  radiation  from  the  sun  must 
suffice  to  convey,  in  each  minute,  to  the  surface  of  a 
sphere  whose  radius  is  93,000,000  miles,  fourteen  units 
of  heat  per  square  foot  of  that  surface.  This  radiation 
comes  from  the  surface  of  the  sun.  It  is  easily  shown 
that  the  heat  from  each  square  foot  on  the  sun  will 
have  to  supply  an  area  of  46,000  square  feet  at  the  dis- 
tance of  the  earth.  Hence  the  number  of  units  of  heat 
emerging  each  minute  from  a  square  foot  on  the  sun's 
surface  must  be  about  640,000. 

We  can  best  realise  what  this  statement  implies 
by  finding  the  amount  of  coal  which  would  produce 
the  same  quantity  of  heat.  It  can  be  shown  that  the 
heat  given  out  by  each  square  foot  of  the  solar  sur- 
face in  one  minute  will  be  equivalent  to  that  pro- 
duced in  the  combustion  of  forty-six  pounds  of  coal. 
If  the  sun's  heat  were  sustained  by  combustion,  every 
part  of  the  sun's  surface  as  large  as  the  grate  of  an 
ordinary  furnace  would  have  to  be  doing  at  least  one 
hundred  times  as  much  heating  as  the  most  vigorous 
stoking  could  extract  from  any  actual  furnace. 


THE    ENERGY   OF    THE   SUN.  83 

The  radiation  of  heat  from  a  single  square  foot  of 
the  solar  surface  in  the  course  of  a  year  must,  there- 
fore, be  equivalent  to  the  heat  generated  in  the  com- 
bustion of  11,000  tons  of  the  best  coal.  If  we  esti- 
mate the  annual  coal  production  of  Great  Britain  at 
250,000,000  tons,  we  find  that  the  total  heat  which 
this  coal  can  produce  is  not  greater  than  the  annual 
emission  from  a  square  of  the  sun's  surface  of  which 
each  side  is  fifty  yards.  All  the  coal  exported  from 
England  in  a  year  does  not  give  as  much  heat  as  the 
sun  radiates  in  the  same  time  from  every  patch  on 
its  surface  which  is  as  big  as  a  croquet  ground. 

There  is  perhaps  no  greater  question  in  the  study 
of  Nature  than  that  which  enquires  how  the  sun's  heat 
is  sustained  so  that  the  radiation  is  still  dispensed 
with  unstinted  liberality.  If  we  are  asked  how  the 
sun  can  be  fed  so  as  to  sustain  this  expenditure,  we 
have  to  explain  that  the  sun  is  not  really  fed.  If, 
then,  it  receives  no  adequate  supplies  of  energy  from 
without,  we  have  to  admit  that  the  sun  must  be 
getting  exhausted. 

I  ought,  indeed,  to  anticipate  objection  by  at  once 
making  the  admission  that  the  sun  does  receive  some 
small  supply  of  energy  from  the  meteors  which  are 
perennially  drawn  into  ft.  The  quantity  of  energy 
they  yield  is,  however,  insignificant  in  comparison  with 
the  "solar  expenditure  of  heat.  We  may  return  to  this 
subject  at  a  later  period,  and  it  need  not  now  receive 
further  attention. 

We  must  deliberately  face  the  fact  that  the  energy 
of  the  sun  is  becoming  exhausted.  But  the  rate  of 
exhaustion  is  so  slow  that  it  affords  no  prospect  of 
inconvenience  to  humanity;  it  does  not  excite  alarm. 


84  THE    EARTH'S    BEGINNING. 

We  grant  that  we  are  not  able  to  observe  by  in- 
strumental means  any  perceptible  diminution  of  solar 
energy.  Still,  as  we  know  that  energy  is  being  steadily 
dissipated  from  the  sun,  and  that  energy  cannot  be 
created  from  nothing,  it  is  certain  the  decline  is  in 
progress.  But  the  reserve  of  energy  which  the  sun 
possesses,  and  which  can  be  ultimately  rendered  avail- 
able to  sustain  the  radiation,  is  so  enormous  in  com- 
parison with  the  annual  expenditure  of  energy,  that 
myriads  of  centuries  will  have  to  elapse  before  there 
is  any  appreciable  alteration  in  the  effectiveness  of 
the  sun. 

Let  me  illustrate  the  point  by  likening  the  sun 
to  a  grain  warehouse,  in  which  2,500  tons  of  wheat 
can  be  accommodated.  Let  us  suppose  that  the  ware- 
house was  quite  full  at  the  beginning,  and  that  the 
wheat  was  to  be  gradually  abstracted,  but  only  at 
the  rate  of  one  grain  each  day.  Let  us  further  sup- 
pose that  no  more  wheat  is  to  be  added  to  that 
already  in  the  warehouse,  and  let  us  assume  that  the 
wheat  thus  stored  away  experiences  no  deterioration 
and  no  loss  whatever  except  by  the  removal  of  one 
grain  per  diem.  It  is  easy  to  see  that  very  many 
centuries  would  have  to  elapse  before  the  grain  in 
that  warehouse  had  decreased  to  any  appreciable 
extent. 

With  a  consumption  at  the  rate  of  a  single  grain 
a  day  a  ton  of  corn  would  last  about  four  thousand 
years,  and  2,500  tons  of  corn  would  accordingly  last 
about  ten  million  years.  It  follows,  therefore,  that  if 
the  grain  in  that  store  were  consumed  at  the  rate  of 
only  one  grain  per  day  the  warehouse  would  not  be 
emptied  for  ten  million  years. 


HOW  MUCH   HEAT   HAS    THE    SUN?  85 


Fig.  15. —  I.  SPECTRUM  OF  THE  SUN. 
II.  SPECTRUM  OF  ARCTURUS. 
(Professor  H.  C.  Lord.) 

The  quantity  of  heat,  or  rather  the  reserve  of 
energy  equivalent  to  heat,  which  still  remains  stored 
up  in  the  sun  bears  to  the  quantity  of  heat  which 
the  sun  radiates  away  in  a  single  day  a  ratio  some- 
thing like  that  which  a  single  grain  of  corn  bears  to 
2,500  tons  of  corn. 

The  sun's  potential  store  of  heat  is  no  doubt  very 
great,  though  not  indefinitely  great.  That  heat  is 
beyond  all  doubt  to  be  ultimately  exhausted ;  but 
the  reserve  is  so  prodigious  that  for  the  myriads  of 
years  during  which  the  sun  has  been  subjected  to 
human  observation  there  has  been  no  appreciable 
alteration  in  the  efficiency  of  radiation. 

It  might  be  supposed  that  the  sun  was  merely  a 
white-hot  globe  cooling  down,  and  that  the  solar  radia- 
tion was  to  be  explained  in  this  way.  But  a  little 
calculation  will  prove  it  to  be  utterly  impossible  that 
the  heat  of  the  great  luminary  could  be  so  accounted 
for.  A  knowledge  of  the  current  expenditure  of  solar 
heat  shows  that  if  the  sun  had  been  a  globe  of  iron 
at  its  fusing  point,  then  at  the  present  rate  of  radiation 
7 


86  THE   EARTH'S^  BEGINNING. 

it  would  have  sunk  to  the  temperature  of  freezing 
water  in  forty-eight  years. 

Perhaps  I  ought  here  to  explain  a  point  which 
might  otherwise  cause  misapprehension.  For  our  ordi- 
nary sources  of  artificial  heat  we,  of  course,  employ 
some  form  of  combustion.  Whenever  combustion 
takes  place  there  is  chemical  union  between  the 
carbon  or  other  fuel,  whatever  it  may  be,  and  the 
oxygen  of  the  atmosphere.  A  certain  quantity  of 
carbon  enters  into  chemical  union  with  a  definite 
quantity  of  oxygen,  and,  as  an  incident  in  the  process, 
a  definite  quantity  of  heat  is  liberated.  So  much  coal, 
for  instance,  requires  for  complete  combustion  so  much 
air,  and,  granted  a  sufficiency  of  air,  the  union  of  the 
carbon  and  hydrogen  in  the  coal  will  give  out  a  cer- 
tain quantity  of  heat  which  may  be  conveniently 
expressed  by  the  number  of  pounds  of  water  which 
that  heat  would  suffice  to  transform  into  steam.  It  is 
necessary  to  observe  that  there  are  definite  numerical 
relations  among  these  quantities.  The  quantity  of  heat 
that  can  be  produced  by  the  combustion  of  a  pound 
of  any  particular  substance  will  depend  upon  the 
nature  of  that  substance. 

As  chemical  combination  is  the  main  source  of  the 
artificial  heat  which  we  employ  for  innumerable  pur- 
poses on  the  earth,  it  seems  proper  to  consider  whether 
it  can  be  any  form  of  chemical  combination  which  con- 
stitutes the  source  of  the  heat  which  the  sun  radiates 
in  such  abundance.  It  is  easy  to  show  that  the  solar 
radiation  cannot  be  thus  sustained.  The  point  to 
which  I  am  now .%  referring  was  very  clearly  illustrated 
by  Helmholtz  in  a  lecture  he  delivered  many  years 
ago  on  the  origin  of  the  planetary  system. 


THE    CAUSE    OF   THE    SUN'S    HEAT.  87 

To  investigate  whether  the  solar  heat  can  be 
attributed  to  chemical  combination,  we  shall  assume 
for  the  moment  that  the  sun  is  composed  of  those 
particular  materials  which  would  produce  the  utmost 
quantity  of  heat  for  a  given  weight ;  in  other  words, 
that  the  sun  is  formed  of  hydrogen  and  oxygen  in 
quantities  having  the  same  ratio  as  that  in  which 
they  should  be  united  to  form  water.  The'  quantity 
of  heat  generated  by  the  union  of  known  weights  of 
oxygen  and  hydrogen  has  been  ascertained,  by  experi- 
ments in  the  laboratory,  to  exceed  that  which  can 
be  generated  by  corresponding  weights  of  any  other 
materials.  We  can  calculate  how  much  of  the  sun's 
mass,  if  thus  constituted,  would  have  to  enter  into 
combination  every  hour  in  order  to  generate  as  much 
heat  as  the  hourly  radiation  of  the  sun.  We  need  not 
here  perform  the  actual  calculation,  but  merely  state 
the  result,  which  is  a  very  remarkable  one.  It  shows 
that  the  heat  arising  from  the  supposed  chemical 
action  would  not  suffice  to  sustain  the  radiation  of 
the  sun  at  its  present  rate  for  more  than  3,000  years. 
Thirty  centuries  is  a  long  time,  no  doubt,  yet  still  we 
must  remember  that  it  is  no  more  than  a  part  even 
of  the  period  known  to  human  history.  If,  indeed,  it 
had  been  by  combustion  that  the  sun's  heat  was  pro- 
duced, then  from  the  beginning  of  the  sun's  career 
as  a  luminous  object  to  its  final  extinction  and  death 
could  not  be  longer  than  3,000  years,  if  we  assumed 
that  its  radiation  was  to  be  uniformly  that  which  it 
now  dispenses. 

But  it  may  be  said  that  we  are  dealing  only  with 
elements  known  to  us  and  with  which  terrestrial 
chemists  are  familiar,  and  it  may  be  urged  that  the 


88  THE    EARTH'S   BEGINNING. 

sun  possibly  contains  materials  whose  chemical  union 
produces  heat  in  much  greater  abundance  than  do 
the  elements  with  which  alone  we  are  acquainted.  But 
this  argument  cannot  be  sustained.  One  of  the  most 
important  discoveries  of  the  last  century,  the  discovery 
which  perhaps  more  than  any  other  has  tended  to 
place  the  nebular  theory  in  an  impregnable  position, 
is  that  which  tells  us  that  the  elements  of  which  the 
sun  is  composed  are  the  same  as  the  elements  of  which 
our  earth  is  made.  We  shall  have  to  refer  to  this  in 
detail  in  a  later  chapter.  We  now  only  make  this 
passing  reference  to  it  in  order  to  dismiss  the  notion 
that  there  can  be  unknown  substances  in  the  sun 
whose  heat  of  combustion  would  be  sufficiently  great 
to  offer  an  explanation  of  the  extraordinary  abundance 
of  solar  radiation. 

There  is  nothing  more  characteristic  of  the  physical 
science  of  the  century  just  closed  than  the  famous 
discovery  of  the  numerical  relation  which  exists  be- 
tween heat  and  energy.  We  are  indebted  to  the 
life-long  labours  of  Joule,  followed  by  those  of  many 
other  investigators,  for  the  accurate  determination  of 
the  fundamental  constant  which  is  known  as  the 
mechanical  equivalent  of  heat.  Joule  showed  that  the 
quantity  of  heat  which  would  suffice  to  raise  one  pound 
of  water  through  a  single  degree  Fahrenheit  was  the 
precise  equivalent  of  the  quantity  of  energy  which 
would  suffice  to  raise  772  pounds  through  a  height  of  one 
foot.  It  would  be  hard  to  say  whether  this  remarkable 
principle  has  had  a  more  profound  effect  on  practical 
engineering  or  on  the  course  of  physical  science. 
In  practical  engineering,  the  knowledge  of  the  me- 
chanical equivalent  of  heat  will  show  the  engineer 


THE    UNIT    OF    HEAT.  89 

the  utmost  amount  of  work  that  could  by  any  con- 
ceivable apparatus  be  extracted  from  the  heat  poten- 
tially contained  in  a  ton  of  coal.  In  the  study  of 
astronomy  the  application  of  the  same  principle  will 
suffice  to  explain  how  the  sun's  heat  has  been  sustained 
for  illimitable  ages. 

It  will  be   convenient   to   commence   with   a  little 


Fig.  16.— BROOKS'  COMET  AND  METEOR  TKAIL. 
(November  13th,   1893.     Exposure  2  hours.) 

(Photographed  by  Professor  E.  E.  Barnard.) 

calculation,  which  will  provide  us  with  a  result  very 
instructive  when  considering  celestial  phenomena  in 
connection  with  energy.  We  have  seen  that  the  unit 
of  heat — for  so  we  term  the  quantity  of  heat  necessary 
to  raise  a  pound  of  water  one  degree — will  suffice,  when 
transformed  into  mechanical  energy,  to  raise  772  pounds 
through  a  single  foot.  This  would,  of  course,  be  pre- 
cisely the  same  thing  as  to  raise  one  pound  through 
772  feet.  Suppose  a  pound  weight  were  carried  up 


90  THE    EARTH'S    BEGINNING. 

772  feet  high  and  were  then  allowed  to  drop.  The 
pound  weight  would  gradually  gather  speed  in  its 
descent,  and,  at  the  moment  when  it  was  just  reaching 
the  earth,  would  be  moving  with  a  speed  of  about 
224  feet  a  second.  We  may  observe  that  the  work 
which  was  done  in  raising  the  body  to  this  height  has 
been  entirely  expended  in  giving  the  body  this  par- 
ticular velocity.  A  weight  of  one  pound,  moving  with 
a  speed  of  224  feet  a  second,  will  therefore  contain,  in 
virtue  of  that  motion,  a  quantity  of  energy  precisely 
equivalent  to  the  unit  of  heat. 

It  is  a  well-known  principle  in  mechanics  that  if 
a  body  be  dropped  from  any  height,  the  velocity  with 
which  it  would  reach  the  ground  is  just  the  velocity 
with  which  the  body  should  be  projected  upwards 
from  the  ground  in  order  to  re-ascend  to  the  height 
from  which  it  fell  (the  resistance  of  the  air  is  here 
overlooked  as  not  having  any  bearing  upon  the  present 
argument).  Thus  we  see  that  a  weight,  moving  with 
a  velocity  of  224  feet  per  second,  contains  within  itself, 
in  virtue  of  its  motion,  energy  adequate  to  make  it 
ascend  against  gravity  to  the  height  of  772  feet.  That 
is  to  say,  this  velocity  in  a  body  of  a  pound  weight  can 
do  for  the  body  precisely  what  the  unit  of  heat  can  do 
for  it;  hence  we  say  that  in  virtue  of  its  movement 
the  body  contains  a  quantity  of  energy  equal  to  the 
energy  in  the  unit  of  heat. 

Let  us  now  carry  our  calculation  a  little  further. 
If  a  pound  of  good  coal  be  burned  with  a  sufficient 
supply  of  oxygen,  and  if  every  precaution  be  taken 
so  that  no  portion  of  the  heat  be  wasted,  it  can  be 
shown  that  the  combustion  of  the  coal  is  sufficient 
to  produce  14,000  units  of  heat.  In  other  words,  the 


THE    ENERGY  IN    COAL.  91 

burning  of  one  pound  of  coal  ought  to  be  able  to  raise 
14,000  pounds  of  water  one  degree,  or  140  pounds  of 
water  a  hundred  degrees,  or  70  pounds  of  water  two 
hundred  degrees.  I  do  not  mean  to  say  that  efficiency 
like  this  will  be  attained  in  the  actual  circumstances  of 
the  combustion  of  coal  in  the  fireplace.  A  pound  of 
coal  does,  no  doubt,  contain  sufficient  heat  to  boil  seven 
gallons  of  water ;  but  it  cannot  be  made  to  effect  this, 
because  the  fireplace  wastes  in  the  most  extravagant 
manner  the  heat  which  the  coal  produces,  so  that  no 
more  than  a  small  fraction  of  that  heat  is  generally 
rendered  available.  But  in  the  cosmical  operations 
with  which  we  shall  be  concerned  we  consider  the 
full  efficiency  of  the  heat ;  and  so  we  take  for  the 
pound  of  coal  its  full  theoretical  equivalent,  namely, 
14,000  thermal  units.  Let  us  now  find  the  quantity 
of  energy  expressed  in  foot-pounds*  to  which  this  will 
correspond.  It  is  obtained  by  multiplying  14,000  units 
of  heat  by  772,  and  we  get  as  the  result  10,808,000. 
That  is  to  say,  a  pound  of  good  coal,  in  virtue  of  the 
fact  that  it  is  combustible  and  will  give  out  heat, 
contains  a  quantity  of  energy  which  is  represented 
by  ten  or  eleven  million  foot-pounds. 

We  now  approach  the  question  in  another  way. 
Let  us  think  of  a  piece  of  coal  in  rapid  motion ;  if  the 
coal  weighed  a  pound,  and  if  it  were  moving  at  224  feet 
a  second,  then  the  energy  it  contains  in  consequence 
of  that  velocity  would,  as  we  have  seen,  correspond  to 
one  thermal  unit.  We  have,  however,  to  suppose  that 
the  piece  of  coal  is  moving  with  a  speed  much  higher 
than  that  just  stated;  and  here  we  should  note  that 

*  A.  foot-pound  is  the  amount  of  energy  required  to  raise  a  pound 
weight  through  a  height  of  one  foot. 


92  THE    EARTH'S   BEGINNING. 

the  energy  which  a  moving  body  possesses,  in  virtue  of 
its  velocity,  increases  very  rapidly  when  the  speed  of 
that  body  increases.  If  the  velocity  of  a  moving  body 
be  doubled,  the  energy  that  it  possesses  increases  four- 
fold. If  the  velocity  of  the  body  be  increased  tenfold, 
then  the  energy  that  it  possesses  will  be  increased  a 
hundredfold.  More  generally,  we  may  say  that  the 
energy  of  a  moving  body  is  proportional  to  the  square 
of  the  velocity  with  which  the  body  is  animated.  Let 
us,  then,  suppose  that  the  piece  of  coal,  weighing  one 
pound,  is  moving  with  a  speed  as  swift  as  a  shot  from 
the  finest  piece  of  artillery,  that  is  to  say,  with  a  speed 
of  2,240  feet  a  second ;  and  as  this  figure  is  ten  times 
224,  it  shows  us  that  the  moving  body  will  then  possess, 
in  virtue  of  its  velocity,  the  equivalent  of  one  hundred 
units  of  heat. 

But  we  have  to  suppose  a  motion  a  good  deal 
more  rapid  than  that  obtained  by  any  artillery;  we 
shall  consider  a  speed  rather  more  than  ten  times  as 
fast.  It  is  easy  to  calculate  that  if  the  piece  of  coal 
which  weighs  a  pound  is  moving  at  the  speed  of  five 
miles  a  second,  the  energy  that  it  would  possess  in 
consequence  of  that  motion  would  approximate  to 
14,000  thermal  units.  In  other  words,  we  come  to 
the  conclusion  that  any  body  moving  with  a  velocity 
of  five  miles  a  second  will  possess,  in  virtue  of  that 
velocity,  a  quantity  of  energy  just  equal  to  the 
energy  which  an  equally  heavy  piece  of  good  coal 
could  produce  if  burnt  in  oxygen,  and  if  every  portion 
of  the  heat  were  utilised. 

It  is  quite  true  that  the  speed  of '  five  miles  a 
second  here  supposed  represents  a  velocity  much  in 
excess  of  any  velocity  with  which  we  are  acquainted 


THE   EARTH'S   ENERGY.  93 

in  the  course  of  ordinary  experience.  It  is  more 
than  ten  times  as  fast  as  the  speed  of  a  rifle  bullet. 
But  a  velocity  of  five  miles  a  second  is  not  at  all 
large  when  we  consider  the  velocities  of  celestial 
bodies.  We  want  this  fact  relating  to  the  energy  in 
a  piece  of  coal  to  be  remembered.  We  shall  find  it 
very  instructive  as  our  subject  develops,  and  therefore 
we  give  some  illustrations  with  reference  to  it. 

The  speed  of  the  earth  as  it  moves  round  the  sun 
is  more  than  eighteen  miles  a  second — that  is  to  say, 
it  is  three  and  a  half  times  the  critical  speed  of  five 
miles.  In  virtue  of  this  speed  the  earth  has  a  corre- 
sponding quantity  of  energy.  To  find  the  equivalent 
of  that  energy  it  must,  as  already  explained,  be  re- 
membered that  the  energy  of  a  moving  body  is  pro- 
portional to  the  square  of  its  velocity ;  it  follows  that 
the  energy  of  the  earth,  due  to  its  motion  round  the 
sun,  must  be  almost  twelve  times  as  great  as  the 
energy  of  the  earth  would  be  if  it  moved  at  the  rate 
of  only  five  miles  a  second.  But,  we  have  already  seen 
that  a  body  with  the  velocity  of  five  miles  a  second 
would,  in  virtue  of  that  motion,  be  endowed  with  a 
quantity  of  energy  equal  to  that  which  would  be  given 
out  by  the  perfect  combustion  of  an  equal  weight  of 
coal.  It  follows,  therefore,  that  this  earth  of  ours, 
solely  in  consequence  of  the  fact  that  it  is  moving  in 
its  orbit  round  the  sun,  is  endowed  with  a  quantity 
of  energy  twelves  times  as  great  as  all  the  energy 
that  would  be  given  out  in  the  combustion  of  a  mass 
of  coal  equal  to  the  earth  in  weight.  This  may  seem 
an  astonishing  statement ;  but  its  truth  is  undoubted. 
If  it  should  happen  that  the  earth  came  into  collision 
with  another  body  by  which  its  velocity  was  stopped, 


94  THE   EARTH'S    BEGINNING. 

the  principle  of  the  conservation  of  energy  tells  us 
that  this  energy,  which  the  earth  has  in  consequence 
of  its  motion,  must  forthwith  be  transformed,  and  the 
form  which  it  will  assume  is  that  of  heat.  Such  a 
collision  would  generate  as  much  heat  as  could  be 
produced  by  the  combustion  of  twelve  globes  of  solid 
coal,  each  as  heavy  as  the  earth.  We  may  indeed  re- 
mark that  the  coal-seams  in  our  earth's  crust  contain, 
in  virtue  of  the  fact  that  they  partake  of  the  earth's 
orbital  motion,  twelve  times  as  much  energy  as  will 
ever  be  produced  by  their  combustion. 

It  can  hardly  be  doubted  that  such  collisions  as  we 
have  here  imagined  do  occasionally  happen  in  some  parts 
of  space.  Those  remarkable  new  stars  which  from  time 
to  time  break  out  derive,  in  all  probability,  their  tem- 
porary lustre  from  collisions  between  bodies  which 
were  previously  non-luminous.  But  we  need  not  go 
so  far  as  inter-stellar  space  for  a  striking  illustration 
of  the  transformation  of  energy  into  heat.  In  the 
pleasing  phenomena  of  shooting  stars  our  own  atmo- 
sphere provides  us  with  beautiful  illustrations  of  the 
same  principle.  The  shooting  star  so  happily  caught 
on  Professor  Barnard's  plate  (Fig.  16)  may  be  cited 
as  an  example. 


CHAPTER  VI. 

HOW  THE   SUN'S   HEAT   IS  MAINTAINED. 

The  Contraction  of  a  Body — Helmholtz  Explained  Sun-heat— Change 
of  a  Mile  every  Eleven  Years  in  the  Sun's  Diameter — Effect  of 
Contraction  on  Temperature — The  Solar  Constant — Limits  to  the 
Solar  Shrinkage — Astronomers  can  Weigh  the  Sun — Density  of  the 
Sun  —  Heat  Developed  by  the  Falling  Together  of  the  Solar 
Materials— Contraction  of  Nebula  to  Form  the  Earth— Heat  Pro- 
duced in  the  Earth's  Contraction — Similar  Calculation  about  the 
Sun — Earth  and  Sun  Contrasted — Heat  Produced  in  the  Solar 
Contraction  from  an  indefinitely  Great  Nebula— The  Coal  Unit 
Employed — Calculation  of  the  Heat  given  out  by  the  Sun. 

THE  law  which  declares  that  a  body  which  gives  out 
heat  must  in  general  submit  to  a  corresponding 
diminution  in  volume  appears,  so  far  as  we  can  judge, 
to  be  one  of  those  laws  which  have  to  be  obeyed  not 
alone  by  bodies  on  which  we  can  experiment,  but  by 
bodies  throughout  the  extent  of  the  universe.  The 
law  which  bids  the  mercury  ascend  the  stem  of  the 
thermometer  when  the  temperature  rises,  and  descend 
when  the  temperature  falls,  affords  the  principle  which 
explains  some  of  the  grandest  phenomena  of  the 
heavens.  Applied  to  the  solar  system  it  declares  that 
as  the  sun,  in  dispensing  its  benefits  to  the  earth  day 


96  THE    EARTH'S    BEGINNING. 

by  day,  has  to  pour  forth  heat,  so  in  like  manner 
must  it  be  diminishing  in  bulk. 

Assuming  that  this  principle  extends  sufficiently 
widely  through  time  and  space,  we  shall  venture  to 
apply  its  consequences  over  the  mighty  spaces  and 
periods  required  for  celestial  evolution.  We  disdain 
to  notice  the  paltry  centuries  or  mere  thousands  of 
years  which  include  that  infinitesimal  trifle  known  as 
human  history.  Our  time  conceptions  must  undergo  a 
vast  extension. 

It  was  Helmholtz  who  first  explained  by  what 
agency  the  sun  is  able  to  continue  its  wonderful 
radiation  of  heat,  notwithstanding  that  it  receives  no 
appreciable  aid  from  chemical  combination.  Helmholtz 
pointed  out  that  inasmuch  as  the  sun  is  pouring  out 
heat  it  must,  like  every  other  cooling  body,  contract 
We  ought  not,  indeed,  to  say  every  cooling  body:  it 
would  be  more  correct  to  say,  every  body  which  is 
giving  out  heat,  for  the  two  things  are  not  neces- 
sarily the  same.  Indeed,  strange  as  it  may  appear,  it 
would  be  quite  possible  that  a  mass  of  gas  should  be 
gaining  in  temperature  even  though  it  were  losing  heat 
all  the  time.  At  first  this  seems  a  paradox,  but  the 
paradox  will  be  explained  if  we  reflect  upon  the 
physical  changes  which  the  gas  undergoes  in  conse- 
quence of  its  contraction. 

Let  us  dwell  for  a  moment  on  the  remarkable  state- 
ment that  the  sun  is  becoming  gradually  smaller.  The 
reduction  required  to  sustain  the  radiation  corresponds 
to  a  diminution  of  the  diameter  by  about  a  mile  every 
eleven  }Tears.  It  may  serve  to  impress  upon  us  the 
fact  of  the  sun's  shrinkage  if  we  will  remember  that  on 
that  auspicious  day  when  Queen  Victoria  came  to  the 


THE    SHRINKING    OF    THE    SUN.  97 

throne  the  sun  had  a  diameter  more  than  five  miles 
greater  than  it  had  at  the  time  when  her  long  and 
glorious  career  was  ended.  The  sun  that  shone  on 
Palestine  at  the  beginning  of  the  present  era  must 
have  had  a  diameter  about  one  hundred  and  seventy 
miles  greater  than  the  sun  which  now  shines  on  the 
Sea  of  Galilee.  This  process  of  reduction  has  been 
going  on  for  ages,  which  from  the  human  point  of 
view  we  may  practically  describe  as  illimitable.  The 
alteration  in  the  sun's  diameter  within  the  period 
covered  by  the  records  of  man's  sway  on  this  earth 
may  be  intrinsically  large ;  it  amounts  no  doubt  to 
several  hundreds  of  miles.  But  in  comparison  with 
the  vast  bulk  of  the  sun  this  change  in  its  magnitude 
is  unimportant.  A  span  of  ten  thousand  years  will 
certainly  include  all  human  history.  Let  us  take  a 
period  which  is  four  times  as  long.  It  is  easy  to  cal- 
culate what  the  diameter  of  the  sun  must  have  been 
forty  thousand  years  ago,  or  what  the  diameter  of  the 
sun  is  to  become  in  the  next  forty  thousand  years. 
Calculated  at  the  rate  we  have  given,  the  alteration 
in  the  sun's  diameter  in  this  period  amounts  to  rather 
less  than  four  thousand  miles.  This  seems  no  doubt 
a  huge  alteration  in  the  dimensions  of  the  orb  of  day. 
We  must,  however,  remember  that  at  the  present 
moment  the  diameter  of  the  sun  is  about  863,000 
miles,  and  that  a  loss  of  four  thousand  miles,  or  there- 
abouts, would  still  leave  a  sun  with  a  diameter  of 
859,000  miles.  There  would  not  be  much  recognisable 
difference  between  two  suns  of  these  different  dimen- 
sions. I  think  I  may  say  that  if  we  could  imagine  two 
suns  in  the  sky  at  the  same  moment,  which  differed 
only  in  the  circumstance  that  one  had  a  diameter 


98  THE   EARTH'S    BEGINNING. 

of  863,000  miles  and  the  other  a  diameter  of  859,000 
miles,  it  would  not  be  possible  without  careful  tele- 
scopic measurement  to  tell  which  of  the  two  was  the 
larger. 

After  a  contraction  has  taken  place  by  loss  of  heat, 
the  heat  that  still  remains  in  the  body  is  contained 
within  a  smaller  volume  than  it  had  originally.  The 
temperature  depends  not  only  on  the  actual  quantity 
of  heat  that  the  mass  of  gas  contains,  but  also  on  the 
volume  through  which  that  quantity  of  heat  is  dif- 
fused. If  there  be  two  equal  weights  of  gas,  and  if 
they  each  have  the  same  absolute  quantity  of  heat, 
but  if  one  of  them  occupies  a  larger  volume  than  the 
other,  then  the  temperature  of  the  gas  in  the  large 
volume  will  not  be  so  high  as  the  temperature  of  the 
gas  in  the  smaller  volume.  This  is  indeed  so  much 
the  case,  that  the  reduction  of  volume  by  the  loss  ot 
heat  may  sometimes  have  a  greater  effect  in  raising 
the  temperature  than  the  very  loss  of  heat  which  pro- 
duced the  contraction  has  in  depressing  it.  On  the 
whole,  therefore,  a  gain  of  temperature  may  be  shown. 
This  is  what,  indeed,  happens  not  unfrequently  in 
celestial  bodies.  The  contraction  having  taken  place, 
the  lesser  quantity  of  heat  still  shows  to  such  advan- 
tage in  the  reduced  volume  of  the  body,  that  no 
decline  of  temperature  will  be  perceptible.  It  may 
happen  that  simultaneously  with  the  decrease  of  heat 
there  is  even  an  increase  of  temperature. 

The  principle  under  consideration  shows  that, 
though  the  sun  is  now  giving  out  heat  copiously,  it 
does  not  necessarily  follow  that  it  must  at  the  same 
time  be  sinking  in  temperature.  As  a  matter  of  fact, 
physicists  do  not  know  what  course  the  temperature 


THE    TEMPERATURE    OF    THE    8 UK  99 

of  the  sun  is  actually  taking  at  this  moment.  The 
sun  may  now  be  precisely  at  the  same  temperature 
at  which  it  stood  a  thousand  years  ago,  or  it  may  be 
cooler,  or  it  may  be  hotter.  In  any  case  it  is  certain 
that  the  change  of  temperature  per  century  is  small,  too 
small,  in  fact,  to  be  decided  in  the  present  state  of 
our  knowledge.  We  cannot  observe  any  change,  and 
to  estimate  the  change  from  mechanical  principles 
would  only  be  possible  if  we  knew  much  more  about 
the  interior  of  the  sun  than  we  know  at  present. 

We  are  forced  to  the  conclusion  that  the  energy 
of  the  sun,  by  which  we  mean  either  its  actual  heat 
or  what  is  equivalent  to  heat,  must  be  continually 
wasting.  A  thousand  years  ago  there  was  more  heat, 
or  its  equivalent,  in  the  sun  than  there  is  at  present. 
But  the  sun  of  a  thousand  years  ago  was  larger 
than  the  sun  that  we  now  have,  and  the  heat,  or  its- 
equivalent,  a  thousand  years  ago  may  not  have  been 
so  effective  in  sustaining  the  temperature  of  the  bigger 
sun  as  the  lesser  quantity  of  heat  is  in  sustaining  the 
temperature  of  the  sun  at  the  present  day.  It  will 
be  noticed  that  the  argument  depends  essentially  on 
the  alteration  of  the  size  of  the  sun.  Of  course  if 
the  orb  of  day  had  been  no  greater  a  thousand  years 
ago  than  it  is  now,  then  the  sun  of  those  early  days 
would  not  only  have  contained  more  heat  than  our 
present  sun,  but  it  must  have  shown  that  it  did  con- 
tain more  heat.  In  other  words,  its  temperature 
would  then  certainly  have  been  greater  than  it  is  at 
present. 

Thus  we  see  the  importance — so  far  as  radiation 
is  concerned — of  the  gradual  shrinking  of  the  sun. 
The  great  orb  of  day  decreases,  and  its  decrease 


100  THE    EARTHS   BEGINNING. 

has  been  estimated  numerically.  We  cannot,  indeed, 
determine  the  rate  of  decrease  by  actual  telescopic 
measurement  of  the  sun's  disc  with  the  micrometer ; 
observations  extending  over  a  period  of  thousands  of 
years  would  be  required  for  this  purpose.  But  from 
knowing  the  daily  expenditure  of  heat  from  the  sun 
it  is  possible  to  calculate  the  amount  by  which  it 
shrinks.  We  cannot  conveniently  explain  the  matter 
fully  in  these  pages.  Those  who  desire  to  see  the 
calculation  will  find  it  in  the  Appendix.  Suffice  it  to 
say  here  that  the  sun's  diameter  diminishes  about 
sixteen  inches  in  every  twenty- four  hours.  This  is 
an  important  conclusion,  for  the  rate  of  contraction 
of  the  solar  diameter  is  one  of  the  most  significant 
magnitudes  relating  to  the  solar  system. 

sit  was  Helmholtz  who  showed  that  the  contrac- 
tion of  the  sun's  diameter  by  sixteen  inches  a  day  is 
sufficient  to  account  for  the  sustentation  of  the  solar 
radiation.  For  immense  periods  of  time  the  heat 
may  be  dispensed  with  practically  unaltered  liberality. 
The  question  then  arises  as  to  what  time-limit  may 
be  assigned  to  the  efficiency  of  our  orb.  Obviously 
the  sun  cannot  go  on  contracting  sixteen  inches  a  day 
indefinitely.  If  that  were  the  case,  a  certain  number 
of  millions  of  years  would  see  it  vanish  altogether.  The 
limit  to  the  capacity  of  the  sun  to  act  as  a  dispenser 
of  light  and  heat  can  be  easily  indicated.  At  present 
the  sun,  in  its  outer  parts  at  all  events,  is  strictly  a 
vaporous  body.  The  telescope  shows  us  nothing  re- 
sembling a  solid  or  a  liquid  globe.  The  sun  seems 
composed  of  gas  in  which  clouds  and  vapours  are 
suspended.  In  the  sun's  centre  the  temperature  is 
probably  very  much  greater  than  any  temperature 


THE    WEIGHT   OF    THE    SUN.  101 

which  can  be  produced  by  artificial  means;  it  would 
doubtless  be  sufficient  not  only  to  melt,  but  even  to 
drive  into  vapour  the  most  refractory  materials.  On 
the  other  hand,  the  enormous  condensing  pressure  to 
which  those  materials  are  submitted  by  the  stupendous 
mass  of  the  sun  will  have  the  effect  of  keeping  them 
together  and  of  compressing  them  to  such  an  extent 
that  the  density  of  the  gas,  if  indeed  we  may  call  it 
gas,  is  probably  as  great  as  the  density  of  any  known 
matter.  The  fact  is  that  the  terms  liquids,  gases, 
and  solids  cease  to  retain  intelligible  distinctions  when 
applied  to  materials  under  such  pressure  as  would  be 
found  in  the  interior  of  the  sun. 

Astronomers  can  weigh  the  sun.  It  may  well  be 
imagined  that  this  is  a  delicate  and  difficult  operation. 
It  can,  however,  be  effected  with  but  little  margin  of 
uncertainty,  and  the  result  is  a  striking  one.  It  serves 
no  useful  purpose  to  express  the  sun's  weight  as  so 
many  myriads  of  tons.  It  is  more  useful  for  our  present 
purpose  to  set  down  the  density  of  the  sun,  that  is  to 
say,  the  ratio  of  the  weight  of  the  orb,  to  that  of  a 
•globe  of  water  of  the  same  size.  This  is  the  useful 
term  in  which  to  consider  the  weight  of  the  sun. 
Astronomers  are  accustomed  to  think  of  the  weight  of 
our  own  earth  in  this  same  fashion,  and  the  result 
shows  that  the  earth  is  rather  more  than  five  times  as 
heavy  as  a  globe  of  water  of  the  same  size.  We  can 
best  appreciate  this  by  stating  that  if  the  earth  were 
made  of  granite,  and  had  throughout  the  density  which 
we  find  granite  to  possess  at  the  surface,  our  globe 
would  be  about  three  times  as  heavy  as  a  globe  of  water 
of  the  same  size.  If,  however,  the  earth  had  been  en- 
tirely made  of  iron,  it  would  be  more  than  seven  times 
8 


102  THE   EARTH'S    BEGINNING. 

as  heavy  as  a  globe  of  water  of  the  same  size.  As  the 
earth  actually  has  a  density  of  5,  it  follows  that  our 
globe  taken  as  a  whole  is  heavier  than  a  globe  of 
granite  of  the  same  size,  though  not  so  heavy  as  a 
globe  of  iron. 

In  the  matter  of  density  there  is  a  remarkable 
contrast  between  the  sun  and  the  earth.  The  sun's 
density  is  much  less  than  that  of  the  earth.  Of  course 
it  will  be  understood  that  the  sun  is  actually  very  much 
heavier  than  our  globe;  it  is  indeed  more  than  three 
hundred  thousand  times  greater  in  weight.  But  the 
sun  is  about  a  million  three  hundred  thousand  times 
as  big  as  the  earth,  and  it  follows  from  these  figures 
that  its  density  cannot  be  more  than  about  a  fourth  of 
that  of  the  earth.  The  result  is  that,  at  present,  the 
sun  is  nearly  half  as  heavy  again  as  a  globe  of  water 
the  same  size.  We  have  used  round  numbers :  the 
density  of  the  sun  is  actually  1'4. 

In  the  following  manner  we  explain  how  heat  is 
evolved  in  the  contraction  of  the  sun.  In  its  early 
days  the  sun,  or  rather  the  materials  which  in  their 
aggregate  form  now  constitute  the  sun,  were  spread 
over  an  immense  tract  of  space,  millions  of  times 
greater  than  the  present  bulk  of  the  sun.  We  see 
nebulosities  even  now  in  the  heavens  which  may 
suggest  what  the  primaeval  nebula  may  have  been 
before  the  evolution  had  made  much  progress.  Look 
for  instance  at  Sir  David  Gill's  photograph  of  the 
Nebula  in  Argo  in  Fig.  17,  or  at  the  Trifid  Nebula 
hi  Fig.  18.  We  may,  indeed,  consider  the  primaeval 
nebula  to  have  been  so  vast  that  particles  from  the 
outside  falling  into  the  position  of  the  present  solar 
surface  would  acquire  that  velocity  of  three  hundred 


HOW    THE    SUN'S   HEAT  IS   DEVELOPED.     103 

and  ninety  miles  a  second  which  we  know  the  attraction 
of  the  sun  is  capable  of  producing  on  an  object  which 
has  fallen  in  from  an  indefinitely  great  distance.  As  these 


Fig.  17. — ARGO  AND  THE  SURROUNDING  STARS  AND  NEBULOSITY. 
(Photographed  by  Sir  David  Gitt,  K.C.B.) 

parts  are  gradually  falling  together  at  the  centre,  there 
will  be  an  enormous  quantity  of  heat  developed  from 
their  concurrence.  Supposing,  for  instance,  that  the 
materials  of  the  sun  were  arranged  in  concentric 
spherical  shells  around  the  centre,  and  imagining 
these  shells  to  be  separated  by  long  intervals,  so  that 
the  whole  material  of  the  sun  would  be  thus  diffused 
over  a  vast  extent,  then  every  pound  weight  in  the 
outermost  shell,  by  the  very  fact  of  its  sinking  down- 


104  THE    EARTH'S    BEGINNING. 

wards  to  the  present  solar  system,  would  acquire  a 
speed  of  390  miles  a  second,  and  this  corresponds  to 
as  much  energy  as  could  be  produced  by  the  burning 
of  three  tons  of  coal.  But  be  the  fall  ever  so  gentle, 
the  great  law  of  the  conservation  of  energy  tells  us 
that  for  the  same  descent,  however  performed,  the 
same  quantity  of  heat  must  be  given  out.  Each 
pound  in  the  outer  shell  would  therefore  give  out  as 
much  heat  as  three  tons  of  coal.  Every  pound  in 
the  other  shells,  by  gradual  descent  into  the  interior, 
would  also  render  its  corresponding  contribution.  It 
then  becomes  easily  intelligible  how,  in  consequence 
of  the  original  diffusion  of  the  materials  of  the  sun 
over  millions  of  times  its  present  volume,  a  vast 
quantity  of  energy  was  available.  As  the  sun  con- 
tracted this  energy  was  turned  into  radiant  heat. 

We  may  anticipate  a  future  chapter  so  far  as  to 
assume  that  there  was  a  time  when  even  this  solid 
earth  of  ours  was  a  nebulous  mass  diffused  through 
space.  We  are  not  concerned  as  to  what  the  tempera- 
ture of  that  nebulous  mass  may  have  been.  We  may 
suppose  it  to  be  any  temperature  we  please.  The 
point  that  we  have  now  to  consider  is  the  quantity 
of  heat  which  is  generated  by  the  contraction  of  the 
nebula.  That  heat  is  produced  in  the  contraction  will 
be  plain  from  what  has  gone  before.  But  we  may  also 
demonstrate  it  in  a  slightly  different  way.  Let  us  take 
any  two  points  in  the  nebula,  P  and  Q.  After  the 
nebula  has  contracted  the  points  which  were  originally 
at  P  and  Q  will  be  found  at  two  other  points,  A  and  B. 
As  the  whole  nebula  in  its  original  form  was  larger 
than  the  nebula  after  it  has  undergone  its  contraction, 
the  distance  P  Q  is  generally  greater  than  the  distance 


CONTRACTION   OF   A    NEBULA.  105 


Fig.  18. — TRIFID  NEBULA  IN  SAGITTARIUS  (Lick  Observatory, 
California). 

(From  the  Royal  Astronomical  Society  Series.) 

A  B.  We  may  suppose  the  contraction  to  proceed 
uniformly,  so  that  the  same  will  be  true  of  the  distance 
between  any  other  two  particles.  The  distance  between 
every  pair  of  particles  in  the  contracted  nebula  will 
be  less  than  the  distance  between  the  same  particles 
in  the  original  nebula. 

If  two  attracting  bodies,  A  and  B,  are  to  be  moved 


106  THE    EARTH'S   BEGINNING. 

further  apart  than  they  were  originally,  force  must  be 
applied  and  work  must  be  done.  We  may  measure 
the  amount  of  that  work  in  foot  pounds,  and  then, 
remembering  that  772  foot  pounds  of  work  are  equiva- 
lent to  the  unit  of  heat,  we  may  express  the  energy 
necessary  to  force  the  two  particles  to  a  greater 
distance  asunder  in  the  equivalent  quantity  of  heat. 
If,  therefore,  we  had  to  restore  the  nebula  from  the 
contracted  state  to  the  original  state,  this  would 
involve  a  forcible  enlargement  of  the  distance  A  B 
between  every  two  particles  to  its  original  value,  P  Q. 
Work  would  be  required  to  do  this  in  every  case,  and 
that  work  might,  as  we  have  explained,  be  expressed 
in  terms  of  its  equivalent  heat  value.  Even  though 
the  temperature  of  the  nebula  is  the  same  in  its 
contracted  state  as  in  its  original  state,  we  see  that  a 
quantity  of  heat  might  be  absorbed  or  rendered  latent 
in  forcing  the  nebula  from  one  condition  to  the 
other.  In  other  words,  keeping  the  temperature  of 
the  nebula  always  constant,  we  should  have  to  apply 
a  large  quantity  of  heat  to  change  the  nebula  from 
its  contracted  form  to  its  expanded  form. 

It  is  equally  true  that  when  the  nebula  is  con- 
tracting, and  when  the  distance  between  every  two 
particles  is  lessening,  the  nebula  must  be  giving  out 
energy,  because  the  total  energy  in  the  contracted 
state  is  less  than  it  was  in  the  expanded  state.  This 
energy  is  equivalent  to  heat.  We  need  not  here 
pause  to  consider  by  what  actual  process  the  heat  is 
manifested;  it  suffices  to  say  that  the  heat  must,  by 
one  of  the  general  laws  of  Nature,  be  produced  in 
some  form. 

We  are  now  able   to  make  a  numerical  estimate. 


FORMATION   OF   HEAT.  107 

We  shall  suppose  that  the  earth,  or  rather  the 
materials  which  make  the  earth,  existed  originally  as 
a  large  nebula  distributed  through  illimitable  space. 
The  calculations  show  that  the  quantity  of  heat,  gener- 
ated by  the  condensation  of  those  materials  from  their 
nebulous  form  into  the  condition  which  the  earth 
now  has,  was  enormously  great.  We  need  not  express 
this  quantity  of  heat  in  ordinary  units.  The  unit  we 
shall  take  is  one  more  suited  to  the  other  dimensions 
involved.  Let  us  suppose  a  globe  of  water  as  heavy 
as  the  earth.  This  globe  would  have  to  be  five  or 
six  times  as  large  as  the  earth.  Next  let  us  realise 
the  quantity  of  heat  that  would  be  required  to  raise 
that  globe  of  water  from  freezing  point  to  boiling 
point.  It  can  be  proved  that  the  heat,  or  its  equiva- 
lent, which  would  be  generated  merely  by  the  con- 
traction of  the  nebula  to  form  the  earth,  would  be 
ninety  times  as  great  as  the  amount  of  heat  which 
would  suffice  to  raise  a  mass  of  water  equal  in 
weight  to  the  earth  from  freezing  point  to  boiling 
point. 

We  apply  similar  calculations  to  the  case  of  the 
sun.  Let  us  suppose  that  the  great  luminary  was 
once  diffused  as  a  nebula  over  an  exceedingly  great 
area  of  space.  It  might  at  first  be  thought  that  the 
figures  we  have  just  given  would  answer  the  question. 
We  might  perhaps  conjecture  that  the  quantity  of 
heat  would  be  such  as  would  raise  a  mass  of  water 
equal  to  the  sun's  mass  from  freezing  to  boiling  point 
ninety  times  over.  But  we  should  be  very  wrong  in 
such  a  determination.  The  heat  that  is  given  out  by 
the  sun's  contraction  is  enormously  greater  than  this 
estimate  would  represent,  and  we  shall  be  prepared  to 


108  THE   EARTH'S   BEGINNING. 

admit  this  if  we  reflect  on  the  following  circumstances. 
A  stone  falling  from  an  indefinitely  great  distance  to 
the  sun  would  acquire  a  speed  of  390  miles  a 
second  by  the  time  it  reached  the  sun's  surface.  A 
stone  falling  from  an  indefinitely  great  distance  in 
space  to  the  earth's  surface  would,  however,  acquire  a 
speed  of  not  more  than  seven  miles  a  second.  The  speed 
acquired  by  a  body  falling  into  the  sun  by  the  gravi- 
tation of  the  sun  is,  therefore,  fifty-six  times  as  great 
as  the  speed  acquired  by  a  body  falling  from  infinity 
to  the  earth  by  the  gravitation  of  the  earth.  As  the 
energy  of  a  moving  body  is  proportional  to  the  square 
of  its  velocity,  we  see  that  the  energy  with  which 
the  falling  body  would  strike  the  sun,  and  the  heat 
that  it  might  consequently  give  forth,  would  be  about 
three  thousand  times  as  great  as  the  heat  which 
would  be  the  result  of  the  fall  of  that  body  to  the 
earth.  We  need  not  therefore  be  surprised  that  the 
drawing  together  of  the  elements  to  form  the  sun 
should  be  accompanied  by  the  evolution  of  a  quantity 
of  heat  which  is  enormously  greater  than  the  mere 
ratio  of  the  masses  of  the  earth  and  sun  would  have 
suggested. 

There  is  another  line  of  reasoning  by  which  we 
may  also  illustrate  the  same  important  principle. 
Owing  to  the  immense  attraction  possessed  by  the 
large  mass  of  the  sun,  the  weights  of  objects  on 
that  luminary  would  be  very  much  greater  than  the 
weights  of  corresponding  objects  here.  Indeed,  a  pound 
on  the  sun  would  be  found  by  a  spring-balance  to 
weigh  as  much  as  twenty-seven  pounds  here.  If  the 
materials  of  the  sun  had  to  be  distributed-  through 
space,  each  pound  lifted  a  foot  would  require  twenty- 


CONDENSATION  AND    HEAT.  109 

seven  times  the  amount  oi  work  which  would  be 
necessary  to  lift  a  pound  through  a  foot  on  the  earth's 
surface.  It  will  thus  be  seen  that  not  only  the  quan- 
tity of  material  that  would  have  to  be  displaced  is 
enormously  greater  in  the  sun  than  in  the  earth,  but 
that  the  actual  energy  that  would  have  to  be  applied 
per  unit  of  mass  from  the  sun  would  be  many  times 
as  great  as  the  quantity  of  energy  that  would  have  to 
be  applied  per  unit  of  mass  from  the  earth  to  effect  a 
displacement  through  the  same  distance.  To  distribute 
the  sun's  materials  into  a  nebula  we  should  therefore 
require  the  expenditure  of  a  quantity  of  work  far 
more  than  proportional  to  the  mere  mass  of  the 
sun.  It  follows  that  when  the  sun  is  contracting  the 
quantity  of  work  that  it  will  give  out,  or,  what  comes 
to  the  same  thing,  the  amount  of  heat  that  would 
be  poured  forth  in  consequence  of  the  contraction 
per  unit  of  mass  of  the  sun  will  largely  exceed  the 
quantity  of  heat  given  out  in  the  similar  contraction 
of  the  earth  per  unit  of  mass  of  the  earth. 

These  considerations  will  prepare  us  to  accept  the 
result  given  by  accurate  calculation.  It  has  been  shown 
that  the  heat  which  would  be  generated  by  the  con- 
densation of  the  sun  from  a  nebula  filling  all  space 
down  to  its  present  bulk  is  two  hundred  and  seventy 
thousand  times  the  amount  of  heat  which  would  be 
required  to  raise  the  temperature  of  a  mass  of  water 
equal  to  the  sun  from  freezing  point  to  boiling  point. 

This  is  a  result  of  a  most  instructive  character. 
The  amount  of  heat  that  would  be  required  to  raise  a 
pound  of  water  from  freezing  point  to  boiling  point 
would,  speaking  generally,  be  quite  enough  if  applied 
to  a  pound  of  stone  or  iron  to  raise  either  of  these 


110  THE   EARTH'S    BEGINNING. 

masses  to  a  red  heat.  If,  therefore,  we  think  of  the 
sun  as  a  mighty  globe  of  stone  or  iron,  the  amount 
of  heat  that  would  be  produced  by  the  contraction 
of  the  sun  from  the  primaeval  nebula  would  suffice 
to  raise  that  globe  of  stone  or  iron  from  freezing 
point  up  to  a  red  heat  270,000  times.  This  will 
give  us  some  idea  of  the  stupendous  amount  of  heat 
which  has  been  placed  at  the  disposal  of  the  solar 
system  by  the  process  of  contraction  of  the  sun. 
This  contraction  is  still  going  on,  and  consequently 
the  yield  of  heat  which  is  the  consequence  of  this 
contraction  is  still  in  progress,  and  the  heat  given 
out  provides  the  annual  supply  necessary  for  the 
sustenance  of  our  solar  system. 

There  is  one  point  which  should  be  specially 
mentioned  in  connection  with  this  argument.  We 
have  here  supposed  that  the  current  supply  of  radiant 
heat  from  the  sun  is  entirely  in  virtue  of  the  sun's 
contraction.  That  is  to  say,  we  suppose  the  sun's 
temperature  to  be  remaining  unaltered.  This  is  perhaps 
not  strictly  the  case.  There  may  be  reason  for  be- 
lieving that  the  temperature  of  the  sun  is  increasing, 
though  not  to  an  appreciable  extent. 

It  will  be  convenient  to  introduce  a  unit  that  will 
be  on  a  scale  adapted  to  our  measurements.  Let  us 
think  of  a  globe  of  coal  as  heavy  as  the  sun.  Now 
suppose  adequate  oxygen  were  supplied  to  burn  that 
coal,  a  definite  quantity  of  heat  would  be  produced. 
There  is  no  present  necessity  to  evaluate  this  in  the 
lesser  units  adapted  for  other  purposes.  In  discussing 
the  heat  of  the  sun,  we  may  use  what  we  call  the 
coal  unit,  by  which  is  to  be  understood  the  total 
quantity  of  heat  that  would  be  produced  if  a  mass 


THE    COAL    UNIT.  Ill 

of  coal  equal  to  the  sun  in  weight  were  burned  in 
oxygen.  It  can  be  shown  by  calculations,  which  will 
be  found  in  the  Appendix,  that  in  the  shrinkage  of 
the  sun  from  an  infinitely  great  extension  through 
space  down  to  its  present  bulk  the  contraction  would 
develop  the  stupendous  quantity  of  heat  represented 
by  3,400  coal  units.  It  is  also  shown  that  one  coal 
unit  would  be  adequate  to  supply  the  sun's  radiation 
at  its  present  rate  for  2,800  years. 


CHAPTER   VII. 

THE  HISTORY   OF  THE   SUN. 

The  Inconstant  Sun— Representation  of  the  Solar  System  at  different 
Epochs — Primaeval  Density  of  the  Sun — Illustration  of  Gas  in 
Extreme  Tenuity — Physical  State  of  the  Sun  at  that  Period — 
The  Sun  was  then  a  Nebula. 

WE  pointed  out  in  the  last  chapter  how,  in  consequence 
of  its  perennial  loss  of  heat,  the  orb  of  day  must  be 
undergoing  a  gradual  diminution  in  size.  In  the  present 
chapter  we  are  to  set  down  the  remarkable  conclusions 
with  respect  to  the  early  history  of  the  sun  to  which  we 
have  been  conducted  by  pursuing  to  its  legitimate  con- 
sequences the  shrinkage  which  the  sun  had  undergone 
in  times  past*. 

The  outer  circle  in  Fig.  19  represents  the  track  in 
which  our  earth  now  revolves  around  the  sun,  and  we 
are  to  understand  that  the  radius  of  this  circle  is  about 
ninety-three  million  miles.  We  must  imagine  that  the 
innermost  of  the  four  circles  represents  the  position  of 
the  sun.  Along  its  track  the  earth  revolves  year  after 
year ;  so  it  has  revolved  for  centuries,  so  it  has  revolved 
since  the  days  of  the  first  monarch  that  ever  held  sway 
in  Britain,  so  it  has  revolved  during  all  the  time  over 
which  history  extends,  so  it  has  doubtless  revolved  for 


THE    CHANGING    SUN. 


113 


Fig.  19.— To  ILLUSTRATE  THE  HISTORY  or  THE  SUN. 

illimitable  periods  anterior  to  history.  For  an  interval 
of  time  that  no  one  presumes  to  define  with  any  accuracy 
the  earth  has  revolved  in  the  same  track  round  that 
sun  in  heaven  which,  during  all  those  ages,  has  dispensed 
its  benefits  of  light  and  heat  for  the  sustenance  of  life 
on  our  globe. 

The  sun  appears  constant  during  those  few  years  in 
which  man  is  allowed  to  strut  his  little  hour.  The  size 
of  the  sun  and  the  lustre  of  the  sun  has  not  appreciably 
altered.  But  the  sun  does  not  always  remain  the  same. 
It  has  not  always  shone  with  the  brightness  and  vigour 
with  which  it  shines  now;  it  will  not  continue  for  ever  to 
dispense  its  benefits  with  the  same  liberality  that  it  does 
at  present.  The  sun  is  always  in  a  state  of  change.  It 


114  THE    EARTH'S   BEGINNING. 

would  not  indeed  be  correct  to  refer  to  these  changes  as 
growths,  in  the  same  sense  in  which  we  speak  of  the 
growth  in  a  tree.  Decade  after  decade  the  tree  waxes 
greater ;  but  the  sun,  as  we  have  already  explained,  does 
nok  increase  with  the  time,  for  the  change  indeed  lies  the 
other  way.  It  may  well  be  that  in  this  present  era  the 
sun  is  near  its  prime,  in  so  far  as  its  capacity  to  radiate 
warmth  and  brightness  is  concerned.  It  is,  however, 
certain  that  the  sun  is  not  now  so  large  as  it  was  in 
ancient  days.  The  diminution  of  the  orb  is  still  in  pro- 
gress. In  these  present  days  of  its  glorious  splendour 
the  orb  of  day  is  much  larger  than  it  will  be  in  that 
gloomy  old  age  which  destiny  assigns  to  it. 

We  have  already  shown  how  to  give  numerical  pre- 
cision to  our  facts.  We  have  stated  that  the  sun's  dia- 
meter is  diminishing  at  the  rate  of  one  mile  every  eleven 
years.  We  have  dwelt  upon  the  remarkable  signifi- 
cance of  that  shrinkage  in  accounting  for  the  sustentation 
of  the  sun's  heat.  We  have  now  to  call  on  this  perennial 
diminution  of  the  sun's  diameter  to  provide  some  in- 
formation as  to  the  early  history  of  our  .luminary . 

The  innermost  circle  in  our  sketch  is  to  suggest  the 
sun  as  it  is  at  present.  Millions  of  years  ago  the  orb  of 
day  was  as  large  as  I  have  indicated  it  by  the  circle  with 
the  words  "  sun  in  very  early  times."  It  will,  of  course, 
be  understood  that  we  do  not  make  any  claim  to  precise 
representation  of  the  magnitude  of  the  orb.  At  a 
period  much  earlier  still,  the  sun  must  have  been  larger 
still,  and  we  venture  so  to  depict  it.  We  know  the 
rate  at  which  the  sun  is  now  contracting,  and  doubtless 
this  rate  has  continued  sensibly  unaltered  during  thou- 
sands of  years,  and  indeed  we  might  say  scores  of 
thousands  of  years.  But  it  would  not  be  at  all  safe  to 


THE    SUN   OF  LONG    AGO.  115 

assume  that  the  annual  rate  of  change  in  the  sun's  radius 
has  remained  the  same  throughout  excessively  remote 
periods  in  its  evolutionary  history.  What  we  do  affirm 
is,  that  in  the  course  of  its  evolution  the  sun  must  have 
been  contracting  continually,  and  we  have  been  able  to 
learn  the  particular  rate  of  contraction  characteristic  of  the 
present  time.  But  though  we  are  ignorant  of  the  rate  of 
contraction  at  very  early  epochs,  yet  the  sun  ever  looms 
larger  and  larger  in  days  earlier  and  still  earlier.  But  in 
those  early  days  the  sun  was  not  heavier,  was  not,  indeed, 
quite  so  heavy  as  it  is  at  present.  For  we  remember 
that  the  sun  is  perennially  adding  thousands  of  tons  to 
its  bulk  by  the  influx  of  meteors.  Perhaps  we  ought  to 
add  that  the  gain  of  mass  from  the  meteors  may  be  to 
some  extent  compensated  by  the  loss  of  substance  which 
the  sun  not  infrequently  experiences  if,  as  is  sometimes 
supposed,  it  expels  in  some  violent  convulsion  a  mass 
of  material  which  takes  the  form  of  a  comet  (Fig.  21). 

Let  us  now  consider  what  the  density  of  the  sun  must 
have  been  in  those  primaeval  days,  say,  for  example, 
when  the  luminary  had  ten  times  the  volume  that  it 
has  at  present.  Even  now,  as  already  stated,  it  does  not 
weigh  half  as  much  again  as  a  globe  of  water  of  the 
same  size,  so  that  when  it  was  ten  times  as  big  its 
density  must  have  been  only 'a  small  fraction  of  that 
of  water.  But  we  may  take  a  stage  still  earlier.  Let  us 
think  of  a  time — it  was,  perhaps,  many  scores  of  millions 
of  years  ago — when  the  sun  was  a  thousand  times  as  big 
as  it  is  at  present.  The  same  quantity  of  matter  which 
now  constitutes  the  sun  was  then  expanded  over  a 
volume  a  thousand  times  greater.  A  remarkable  con- 
clusion follows  from  this  consideration.  The  air  that 
we  breathe  has  a  density  which  is  about  the  seven- 


116  THE   EARTH'S    BEGINNING. 

hundredth  part  of  that  of  water.  Hence  we  see  that  at 
the  time  when  the  materials  of  the  sun  were  expanded 
into  a  volume  a  thousand  times  as  great  as  it  is  at 
present  the  density  of  the  luminary  must  have  been 
about  equal  to  that  of  ordinary  air.  We  refer,  of  course, 
in  such  statements  to  the  average  density  of  the  sun. 
It  will  be  remembered  that  the  density  of  the  sun  cannot 
be  uniform.  The  mutual  attractions  and  pressures  of 
the  particles  in  the  interior  must  make  the  density 
greater  the  nearer  we  approach  to  the  centre. 

We  must  push  our  argument  further  still.  We 
have  ascertained  that  the  primaeval  sun  could  not 
have  been  a  dense  solid  body  like  a  ball  of  metal. 
It  must  have  been  more  nearly  represented  by  a 
ball  of  gas.  There  was  a  time  when  that  collection 
of  matter  which  now  constitutes  the  sun  was  so  big 
that  a  balloon  of  equal  size,  filled  at  ordinary  pres- 
sure with  the  lightest  of  known  gases,  would  contain 
within  it  a  heavier  weight  than  the  sun.  At  this 
early  period  the  sun  must  have  been  as  light  as  an 
equal  volume  of  hydrogen.  The  reasoning  which  has 
conducted  us  to  this  point  remains  still  unimpaired. 
From  that  early  period  we  may  therefore  look  back 
to  periods  earlier  still.  We  see  that  the  sun  must 
have  been  ever  larger  and  larger,  for  the  same 
quantity  of  material  must  have  been  ever  more  and 
more  diffused.  There  was  a  time  when  the  mean 
density  of  the  sun  must  have  been  far  less  than 
that  of  the  gas  in  any  balloon. 

We  must  not  pause  to  consider  intermediate 
stages.  We  shall  look  back  at  once  to  an  exces- 
sively early  period  when  the  sun — or  perhaps  we 
ought  rather  to  say  the  matter  which  in  a  more 


IN  PRIMEVAL    TIMES. 


117 


Fig.  20.— THE  SOLAR  CORONA  (January  1st,  1899). 
(Photographed   during    Eclipse    by    Professor    W.    H.    Pickering.) 

condensed  form  now  constitutes  the  sun — was  ex- 
panded throughout  the  volume  of  a  globe  whose 
radius  was  as  great  as  the  present  distance  from  the 
sun  to  the  earth.  Have  we  not  here  truly  an  as- 
tonishing result,  deduced  as  a  necessary  consequence 
from  the  fundamental  laws  of  heat  ? 

I  need  hardly  say  that  the  sun  at  that  early  date 
did  not  at  all  resemble  the  glorious  orb  to  which 
we  owe  our  very  existence.  The  primaeval  sun  must 
have  been  a  totally  different  object,  as  we  can 
easily  imagine  if  we  try  to  think  that  the  sun's 
9 


118  THE    EARTH'S    BEGINNING. 

materials  then  filled  a  volume  twelve  million  times 
as  great  as  they  occupy  at  present.  Instead  of  com- 
paring such  an  object  with  the  gases  in  our  ordinary 
atmosphere,  it  should  rather  be  likened  to  the  residue 
left  in  an  exhausted  receiver  after  the  resources  of 
chemistry  have  been  taxed  to  make  as  near  an  ap- 
proach as  possible  to  a  perfect  vacuum. 

We  can  give  a  familiar  -illustration  of  gas  in  a 
state  of  extreme  tenuity.  Look  at  the  beautiful 
incandescent  light  with  which  in  these  days  our 
buildings  are  illuminated.  How  brilliantly  those 
little  globes  shine !  The  globe  has  to  be  most  care- 
fully sealed  against  the  outside  air.  If  there  were 
the  smallest  opportunity  for  access,  the  air  from 
outside  would  rush  in  and  the  lamp  would  be  de- 
stroyed. In  the  preparation  of  such  a  lamp  elaborate 
precautions  have  to  be  taken  to  secure  that  the  ex- 
haustion of  the  air  from  the  little  globe  shall  be 
as  nearly  perfect  as  possible.  Of  course  it  is  impos- 
sible to  remove  all  the  air.  No  known  processes 
can  produce  a  perfect  vacuum.  Some  traces  of  gas 
would  remain  after  the  air-pump  had  been  applied 
even  for  hours. 

We  must  now  imagine  a  globe,  not  merely  two- 
inches  in  diameter  like  one  of  these  little  lamps,  but 
a  globe  186,000,000  miles  in  diameter,  a  globe  so 
large  that  the  earth's  orbit  would  just  form  a  girdle 
round  it.  Even  if  this  globe  had  been  exhausted,  so 
that  its  density  was  only  the  twelve-thousandth  part 
of  the  ordinary  atmospheric  density,  it  would  still 
contain  more  material  than  is  found  in  the  sun  in 
heaven.  Thus  our  reasoning  has  conducted  us  to 
the  notion  of  an  epoch  when  the  sun — or  rather  I 


THE    SUN    WITHOUT   LIGHT   AND    HEAT.       119 


Fig.  21.— THE  GREAT  COMET  OF  1882. 
(Photographed  on  November  7th,  1882,  by  Sir  David  Gill,  K.C.B.) 

should  say  the  matter  composing  the  sun — formed 
something  totally  different  from  the  orb  which  we 
know  so  well.  The  matter  in  that  very  diffuse 
state  would  not  dispense  light  and  heat  as  a  sun 
in  the  sense  in  which  we  understand  the  word. 
However  vast  might  be  the  store  of  energy  which 
it  contained  —  a  store  indeed  thousands  of  times 
greater  than  our  present  sun  possesses — yet  it  would 
hardly  be  possessed  of  the  power  of  effective  radia- 
tion. It  would  assuredly  not  be  able  to  warm  and 
light  a  world  associated  with  it,  in  the  same  way  as 
the  sun  now  provides  so  gloriously  for  our  wants  and 
comfort. 

But  it  is  certain  that  in  those  early  days  there  was 
no  earth  to  be  warmed  and  lighted.      Our  globe,  even  if 


120  THE   EARTH'S    BEGINNING. 

it  can  be  said  to  have  existed  at  all,  was  truly  "  without 
form  and  void."  At  the  time  when  the  sun  was 
swollen  into  a  great  globe  of  gas  or  rarefied  matter, 
the  elementary  substances  which  were  to  form  the 
future  earth  were  in  a  condition  utterly  different  from 
that  of  our  present  globe.  The  history  of  this  earth 
itself  involves  another  chapter  of  the  argument.  Let  it 
suffice  to  notice,  for  the  present,  that  our  reasoning 
has  led  us  to  a  time  when  the  sun  consisted  only 
of  a  rarefied  gaseous  material,  and  let  us  give  to  the 
matter  in  this  condition  the  name  which  astronomers 
apply  to  any  object  of  a  similar  character  wherever 
they  may  meet  with  it  in  the  universe.  Suppose 
that  we  could  observe  through  our  telescopes  at  the 
present  moment  an  object  in  remote  space  which  was 
like  what  the  sun  must  have  been  at  that  early  stage 
of  its  existence  which  we  have  been  considering,  I 
do  not  think  that  the  object  would  be  unfamiliar  to 
astronomers.  There  is,  indeed,  no  doubt  that  there 
are  many  objects  visible  at  this  moment,  and  nightly 
studied  in  our  observatories,  which  are  formed  of 
matter  just  in  the  same  state  as  the  sun  was  in 
those  early  times.  Examined  with  a  good  telescope, 
the  object  would  seem  like  a  small  stain  of  light 
on  the  black  background  of  the  sky.  The  observer 
would  at  once  call  it  a  nebula.  In  these  modern 
days  he  would  probably  apply  the  spectroscope  to  it, 
and  this  instrument  would  assure  him  that  the  object 
he  was  looking  at  was  a  mass  of  incandescent  gas. 
Such  an  object  would  in  all  probability  not  greatly 
differ  from  many  nebulae  now  known  to  us. 

This  being  so,  why  should  we  withhold  from  the  sun 
of  primitive  days  the  designation  to  which  it.  seems  to 


THE    SUN  A   NEBULA.  121 

be  so  fully  entitled  ?  Why  should  we  not  speak  of  it  as 
a  nebula  ?  The  application  of  the  laws  of  heat  has 
shown  that  the  great  orb  of  day  was  once  one  of  those 
numerous  objects  which  astronomers  know  as  nebulae, 
and  perhaps  it  may  not  be  too  fanciful  to  suppose 
that  a  trace  of  the  primaeval  nebula  still  survives  in 
what  we  call  the  Solar  Corona  (Fig.  20). 


CHAPTER   VIII. 
THE  EARTH'S  BEGINNING. 

The  Earth  to  be  Studied— A  great  Experiment— The  Diamond  Drill — 
A  Boring  upwards  of  a  Mile  Deep— A  Mechanical  Feat — The 
Scientific  Importance  of  the  Work — Increase  of  Temperature  with 
the  Depth — A  special  Form  of  Thermometer— Taking  the  Tem- 
perature in  the  Boring— The  Level  of  Constant  Temperature — The 
Rate  of  Increase  of  Temperature  with  the  Depth — One  degree 
Fahrenheit  for  every  Sixty-six  Feet  in  Depth — Temperatures  at 
Depths  ahove  a  Mile — Conclusions  as  to  the  Heat  at  very  great 
Depths  —  The  Heat  developed  by  Tidal  Action  —  This  will  not 
account  for  the  Earth's  Internal  Heat — The  Earth  must  be  con- 
tinually Cooling — Inferences  from  the  incessant  loss  of  Heat  from 
the  Earth  —  The  Earth's  Surface  once  Red  Hot,  or  Molten  —  The 
Earth  must  have  originated  from  a  Nebula — The  Earth's  Beginning. 

IN  the  last  chapter  we  endeavoured  to  ascertain  what 
can  be  learned  from  the  radiation  of  the  sun  with 
regard  to  the  history  of  the  solar  system.  In  this 
chapter  we  shall  not  consider  any  body  in  the 
heavens,  but  the  condition  of  the  earth  itself.  We 
have  learned  something  of  the  history  of  the  solar 
system  from  the  celestial  bodies ;  we  shall  now  learn 
something  about  it  in  another  way — from  the  condition 
of  our  globe  at  depths  far  beneath  our  feet. 

It  will  be  convenient  to  commence  by  mentioning 
a  remarkable  experiment  which  was  made  a  few  years 


AN  INTERESTING    EXPERIMENT.  123 

ago.  Though  that  experiment  is  of  great  scientific 
interest,  yet  it  was  not  designed  with  any  scientific 
object  in  view.  Not  less  than  £10,000  was  expended 
on  the  enterprise,  and  probably  so  large  a  sum  has 
never  been  expended  on  a  single  experiment  of  which 
the  sole  object  was  to  add  to  scientific  knowledge. 
In  the  present  case  the  immediate  object  in  view  was, 
of  course,  a  commercial  one.  There  was,  it  may  be 
presumed,  reasonable  expectation  that  the  great  initial 
cost,  and  a  handsome  profit  as  well,  would  be  returned 
as  the  fruits  of  the  enterprise.  Whether  the  great 
experiment  was  successful  from  the  money-making 
point  of  view  does  not  now  concern  us,  but  it  does 
concern  us  to  know  that  the  experiment  was  very 
successful  in  the  sense  that  it  incidentally  afforded 
scientific  information  of  the  very  highest  value. 

The  experiment  in  question  was  made  in  Germany, 
at  Schladebach,  about  fifteen  miles  from  Leipzig.  It 
was  undertaken  in  making  a  search  for  coal.  Some 
enterprising  capitalists  consulted  the  geologists  as  to 
whether  coal-seams  were  likely  to  be  found  in  this 
locality.  They  were  assured  that  coal  was  there, 
though  it  must  certainly  be  a  very  long  way  down, 
and  consequently  the  pit  by  which  alone  the  seams 
could  be  worked  would  have  to  be  unusually  deep. 
The  capitalists  were  not  daunted  by  this  consideration. 
But,  before  incurring  the  great  expense  of  sinking  the 
shaft,  they  determined  to  make  a  preliminary  search 
and  verify  the  actual  presence  of  workable  seams  of 
useful  fuel.  They  determined  to  bore  a  hole  down 
through  the  rocks  deep  enough  to  reach  the  coal,  if 
it  could  be  reached.  A  boring  for  coal  was,  of  course, 
bv  no  means  a  novelty;  but  there  was  an  unpre- 


124  THE   EARTH'S   BEGINNING. 

cedented  degree  of  mechanical  skill  and  scientific 
acumen  shown  in  this  memorable  boring  near  Leipzig. 
The  result  of  this  enterprise  was  to  make  the  deepest 
hole  which,  with  perhaps  a  single  more  recent  exception 
not  of  so  much  scientific  interest,  has  ever  been  pierced 
through  the  crust  of  the  earth.  This  boring  was 
merely  a  preliminary  to  the  operations  which  would 
follow  if  the  experiment  were  successful  in  discovering 
coal.  It  was  accordingly  only  necessary  to  make  a 
hole  large  enough  to  allow  specimens  of  the  strata  to 
be  brought  to  the  surface. 

The  instrument  employed  in  sinking  a  hole  ot  such 
a  phenomenal  depth  through  solid  rock  is  character- 
istic of  modern  enterprise.  The  boring  tool  had  a 
cutting  edge  of  diamonds :  for  no  other  cutting  im- 
plement is  at  once  hard  enough  and  durable  enough 
to  advance  steadily,  yard  by  yard,  through  the  various 
rocks  and  minerals  that  are  met  with  in  the  descent 
through  the  earth's  crust.  We  might,  perhaps,  illus- 
trate the  actual  form  of  the  tool  as  follows:  imagine 
a  piece  of  iron  pipe,  about  six  inches  in  diameter,  cut 
squarely  across,  with  diamonds  inserted  round  its 
circular  end,  and  we  have  a  notion  of  the  diamond 
drill.  If  the  drill  be  made  to  revolve  when  held 
vertically,  with  the  diamonds  in  contact  with  the  rocks, 
the  cutting  will  commence.  As  the  rotation  is  con- 
tinued, the  drill  advances  through  the  rocks,  and  a 
solid  core  of  the  material  will  occupy  the  hollow  of  the 
pipe.  We  do  not  now  enter  into  any  description  of 
the  many  mechanical  details ;  there  are  ingenious  con- 
trivances for  removing  the  debris  produced  by  the 
attrition  of  the  rocks  as  the  diamonds  cut  their  way, 
and  provision  is  also  made  for  carefully  raising  the 


A    GREAT   BORE.  125 

valuable  core  which,  as  it  provides  specimens  of  the 
different  strata  pierced,  will  show  the  coal,  if  coal  is 
ever  reached.  There  is,  of  course,  an  arrangement  by 
which,  as  the  first  length  of  drill  becomes  buried, 
successive  lengths  can  be  added,  so  as  to  transmit  the 
motion  to  the  cutting  edge  and  enable  the  tool  to  be 
raised  when  necessary ;  in  this  manner  one  length  of 
solid  rock  after  another  is  brought  up  for  examination. 
These  cores,  when  ranged  in  series,  give  to  the  miner 
the  information  he  requires  as  to  the  different  beds 
of  rock  through  which  the  instrument  has  pierced  in 
its  descent  and  as  to  the  depths  of  the  beds.  A 
series  of  cores  will  sometimes  show  astonishing  variety 
in  the  material  through  which  the  drill  has  passed. 
Here  the  tool  will  be  seen  passing  through  a  bed  of 
hard  limestone,  and  then  entering  a  bed  of  soft  shale ; 
now  the  tool  bores  through  dense  and  hard  masses 
of  greenstone,  anon  it  pierces,  it  may  be,  a  stratum  of 
white  marble;  and  finally  the  explorer  may  hope  to 
find  his  expectations  realised  by  the  arrival  at  the 
surface  of  a  cylinder  of  solid  coal. 

The  famous  boring  to  which  we  are  now  referring, 
though  very  deep,  was  not  large  in  diameter.  As  it 
descended  the  comparatively  large  tool  first  employed 
was  replaced  by  a  succession  of  smaller  tools,  so  that 
the  hole  gradually  tapered  from  the  surface  to  the 
lowest  point.  At  its  greatest  depth  the  hole  was  indeed 
hardly  larger  than  a  man's  little  finger.  It  increased 
gradually  all  the  way  to  the  surface,  where  it  was  large 
enough  for  a  man's  arm  to  enter  it  easily. 

How  often  do  we  find  that  the  success  which 
rewards  mechanical  enterprise  greatly  transcends  even 
the  most  sanguine  estimate  previously  formed !  Without 


126  THE    EARTH'S    BEGINNING. 

the  actual  experience  which  has  been  acquired,  I  do 
not  think  anyone  could  have  anticipated  the  extra- 
ordinary facilities  which  the  diamond  drill  has  given 
in  the  operations  of  a  deep  boring.  This  hole  at 
Schladebach  was,  indeed,  a  wonderful  success.  It 
pierced  deeper  than  any  previous  excavation,  deeper 
than  any  well,  deeper  than  any  coal  pit.  From  the 
surface  of  the  ground,  where  the  hole  was  some  six 
inches  in  diameter,  down  to  the  lowest  point,  where  it 
was  only  as  large  as  a  little  finger,  the  vertical  depth 
was  not  less  than  one  mile  and  a  hundred  and  seven- 
teen yards. 

It  is  worth  pondering  for  a  moment  on  the  extra.- 
ordinary  mechanical  feat  which  this  represents.  When 
the  greatest  depth  was  reached,  the  total  length  of  the 
series  of  boring  rods  from  the  surface  where  the 
machinery  was  engaged  in  rotating  the  tool  down  to 
the  cutting  diamonds  at  the  lower  end  where  the 
penetration  was  being  effected,  was  as  long  as  from 
Piccadilly  Circus  to  the  top  of  Portland  Place.  If  a 
hole  of  equal  length  had  been  bored  downwards  from 
the  top  of  Ben  Nevis,  it  would  have  reached  the  sea 
level  and  gone  down  1,200  feet  lower  still.  When  the 
foreman  in  charge  wished  to  look  at  the  tool  to  see 
whether  it  was  working  satisfactorily,  or  whether  any 
of  the  diamonds  had  got  injured  or  displaced,  it  was 
necessary  to  raise  that  tremendous  series  of  rods. 
Each  one  of  them  had  to  be  lifted,  had  to  be  un- 
coupled, and  had  to  be  laid  aside.  I  need  hardly  say 
that  such  an  operation  was  a  very  tedious  one.  The 
collective  weight  of  the  working  system  of  rods  was 
about  twenty  tons,  and  not  less  than  ten  hours'  hard 
work  was  required  before  the  tool  was  raised  from  tho 


A    VALUABLE    HOLE.  127 

bottom  to  the  surface.  We  may,  I  believe,  conclude 
that  so  much  ingenuity  and  so  much  trouble  was 
never  before  expended  on  the  act  of  boring  a  hole; 
but  the  results  are  full  of  information  on  important 
problems  of  science. 

I  am  not  going  to  speak  of  the  geological  results 
of  this  exploration.  There  is  not  the  least  doubt  that 
the  remarkable  section  of  the  earth's  crust  thus  ob- 
tained is  of  much  interest  to  geologists.  Our  object 
in  now  alluding  to  this  wonderful  boring  is,  however, 
very  different.  Its  significance  will  be  realised  when 
we  say  that  it  gives  us  more  full  and  definite  informa- 
tion about  the  internal  heat  of  the  earth  than  had 
ever  been  obtained  by  any  other  experiment  on  the 
earth's  crust.  No  doubt  many  previous  observations 
of  the  internal  heat  of  the  globe  were  well  known  to 
the  investigators  who  feel  an  interest  in  these  im- 
portant questions ;  but  the  exceptional  depth  of  this 
boring,  as  well  as  the  exceptionally  favourable  condi- 
tions under  which  it  was  made,  have  rendered  the 
information  derived  from  it  of  the  utmost  value  to 
science. 

We  ought  first  to  record  our  special  obligation  to 
the  German  engineer,  Captain  Huyssen,  who  bored  this 
wonderful  hole.  He  was  not  only  a  highly  skilful 
mining  engineer,  diligent  in  the  pursuit  of  his  pro- 
fession, but,  by  the  valuable  scientific  work  he  has 
done,  he  has  shown  himself  to  be  one  of  those  culti- 
vated and  thoughtful  students  who  love  to  avail 
themselves  of  every  opportunity  of  searching  into 
Nature's  secrets  Our  thanks  are  due  to  him  for 
the  remarkable  zeal  with  which  he  utilised  the  excep- 
tional opportunities  for  valuable  scientific  work  that 


128  THE    EARTH'S    BEGINNING. 

arose,  incidentally  as  it  were,  in  connection  with  the 
work  committed  to  him. 

Of  course,  everybody  knows  that  the  temperature  of 
the  earth  is  found  to  increase  gradually  as  greater  depths 
are  reached.  The  rate  at  which  the  increase  takes  place 
has  been  determined  on  many  occasions.  But  when 
opportunities  have  arisen  for  taking  the  temperature 
at  considerable  depths  below  the  earth's  surface,  it 
has  happened  sometimes  that  the  observations  have 
been  complicated  by  circumstances  which  deprived 
them  of  a  good  deal  of  their  accuracy.  If  our  object 
be  to  learn  the  law  connecting  the  earth's  temperature 
with  the  depth  below  the  surface,  it  is  not  sufficient 
to  study  the  thermometric  readings  in  different  coal 
pits.  Throughout  the  workings  in  every  pit  there 
must  be  arrangements  for  ventilation.  The  cool  air 
has  to  be  drawn  down,  and  thus  the  temperature 
indicated  in  the  pit  is  forced  below  the  temperature 
which  would  really  be  found  at  that  depth  if  external 
sources  of  change  of  temperature  were  absent. 

Captain  Huyssen  rightly  deemed  that  the  hole 
which  he  had  pierced  presented  exceptional  oppor- 
tunities for  the  study  of  the  important  question  of 
the  earth's  internal  temperature.  Precautions  had,  of 
course,  to  be  observed.  The  hole,  as  might  be  ex- 
pected, was  filled  with  water,  and  the  water  would 
tend,  if  its  circulation  were  permitted,  to  equalise  the 
temperature  at  different  depths.  But  the  ingenious 
Captain  quickly  found  an  efficient  remedy  for  this 
source  of  inaccuracy.  He  devised  an  arrangement, 
which  I  must  not  delay  to  describe,  by  which  he 
could  place  temporary  plugs  in  the  hole  at  any  depths 
he  might  desire;  he  then  determined  the  temperature 


A    SPECIAL    THERMOMETER.  129 

of  the  water  in  a  short  length,  so  plugged  above  and 
below  that  the  circulation  was  stopped,  and  accord- 
ingly the  water  thus  confined  might  be  relied  on  to 
indicate  the  temperatures  of  the  strata  which  held  it. 

The  thermometer  employed  in  an  in- 
vestigation of  this  sort  is  ingenious  though 
extremely  simple.  The  ordinary  maximum 
thermometer  is  not  found  to  be  adapted 
for  the  purpose.  The  instrument  (Fig.  22) 
employed  in  the  determination  of  under- 
ground temperatures  is  very  much  less 
complicated  and  at  the  same  time  much 
more  accurate.  The  contrivance  is  indeed 
so  worthy  of  notice  that  I  do  not  like  to 
pass  it  by  without  a  few  words.  The 
thermometer  with  which  the  temperature 
of  the  earth  is  ascertained  in  such  investi- 
gations is  not  like  any  ordinary  ther- 
mometer. There  is  no  scale  of  degrees  Fig.22.— SPECIAL 
attached  to  it  or  engraved  upon  it,  as  we  THERMOMETER 
generally  find  in  such  instruments.  The  !°R  USE  IN 

.  -11-11  f       -L>EEP  BOKINGS. 

instrument  with  which  the  temperature  of 
the  deep  hole  was  measured  was  merely  a  bulb  of  glass 
with  a  slender  capillary  stem,  the  end  of  which  was  not 
closed.  When  it  was  about  to  be  lowered  to  test  the 
temperature  of  the  rocks  at  the  lowest  point  to  which 
the  drill  had  penetrated,  the  bulb  and  the  tube  were 
first  filled  with  mercury  to  the  top,  and  brimming 
over.  This  simple  apparatus  was  attached  to  a  long 
wire,  by  the  aid  of  which  it  could  be  lowered  down 
this  deep  hole.  Down  it  went  till  at  last  the  ther- 
mometer reached  the  bottom,  which,  as  we  have 
explained,  it  could  not  do  until  more  than  a  mile  of 


130  THE    EARTH'S    BEGINNING. 

wire  had  been  paid  out.  The  instrument  was  then 
left  quietly  until  it  presently  assumed  the  same  tem- 
perature as  the  rocks  about  it.  There  could  be  no 
interference  by  heat  from  other  strata,  as  the  circu- 
lation of  water  was  prevented  by  the  plugging  already 
referred  to.  The  temperature  to  which  the  thermo- 
meter had  been  exposed  must,  therefore,  have  been 
precisely  the  temperature  corresponding  in  that  par- 
ticular locality  to  that  particular  depth  below  the 
earth's  surface. 

As  the  thermometer  descended,  it  passed  through 
a  succession  of  strata  of  ever-increasing  temperature. 
Consequently  the  mercury,  which,  it  will  be  remem- 
bered, had  completely  filled  the  instrument  when  it 
was  at  the  surface,  began  to  expand  according  as  it 
was  exposed  to  greater  temperatures.  As  the  mercury 
expanded,  it  must,  of  course,  flow  'out  of  the  tube  and 
be  lost,  because  the  tube  had  been  already  full.  So 
long  as  the  mercury  was  gaining  in  temperature,  more 
and  more  of  it  escaped  from  the  top  of  the  tube,  and 
the  flow  only  ceased  when  the  instrument  was  resting 
at  the  bottom  of  the  hole,  and  the  mercury  became 
as  hot  as  the  surrounding  rocks.  No  more  mercury 
was  then  expelled,  the  tube,  however,  remaining  full 
to  the  brim.  After  allowing  a  sufficient  time  for  the 
temperature  to  settle  definitely,  the  thermometer  was 
raised  to  the  surface.  As  it  ascended  through  the 
long  bore  the  temperature  surrounding  it  steadily 
declined.  With  the  fall  in  the  temperature  of  the 
mercury  the  volume  of  that  liquid  began  to  shrink; 
but  the  mercury  already  expelled  could  not  be  re- 
called. When  at  last  the  instrument  had  safely 
reached  the  surface,  after  its  long  journey  down  and 


HOW    THE    THERMOMETER    WORKED.  131 

up,  and  when  the  mercury  had  regained  the  tempera- 
ture of  the  air,  the  lessened  quantity  that  remained 
told  the  tale  of  the  changes  of  temperature. 

It  is  now  easy  to  see  how,  even  in  the  absence  of 
an  engraved  scale  on  the  instrument,  it  is  possible  to 
determine,  from  the  amount  of  mercury  remaining,  the 
temperature  to  which  the  thermometer  has  been  sub- 
jected at  the  bottom  of  the  boring.  It  is  only 
necessary  to  place  this  thermometer  in  a  basin  of 
cold  water,  and  then  gradually  increase  the  tempera- 
ture by  adding  hot  water.  As  the  temperature 
increases  the  mercury  will,  of  course,  rise,  and  the 
hotter  the  water  the  more  nearly  will  the  mercury 
approach  the  top  of  the  tube.  At  last,  when  the 
mercury  has  just  reached  the  top  of  the  tube,  and 
when  it  is  just  on  the  point  of  overflowing,  we  may 
feel  certain  that  the  temperature  of  the  water  in  the 
basin  has  been  raised  to  the  same  temperature  as  that 
to  which  the  instrument  was  subjected  at  the  bottom 
of  the  boring.  In  each  case  the  temperature  is  just 
sufficient  to  expand  the  quantity  of  mercury  remain- 
ing in  the  instrument  so  as  to  make  it  fill  precisely 
both  bulb  and  stem.  When  this  critical  condition 
is  reached,  it  only  remains  to  dip  a  standard  ther- 
mometer, furnished  with  the  ordinary  graduation, 
into  the  hot  water  of  the  basin.  Thus  we  learn  the 
temperature  of  the  basin,  thus  we  learn  the  temper- 
ature of  the  mercury  in  the  thermometer,  and  thus 
we  determine  the  temperature  at  the  bottom  of  the 
boring  over  a  mile  deep. 

I  need  not  specify  the  details  of  the  arrangements 
which  enabled  the  skilful  engineer  also  to  determine 
the  temperature  at  various  points  of  the  hole  inter- 


132  THE   EARTH'S   BEGINNING. 

mediate  between  the  top  and  the  bottom.  In  fact, 
taking  every  precaution  to  secure  accuracy,  he  made 
measurements  of  the  temperature  at  a  succession  of 
points  about  a  hundred  feet  distant  throughout  the 
whole  depth.  In  each  case  he  was  careful,  as  I  have 
already  indicated,  to  plug  the  hole  above  and  below  the 
thermometer,  so  as  to  prevent  the  circulation  of  water 
in  the  vicinity  of  the  instrument.  The  thermometer, 
therefore,  recorded  the  temperature  of  the  surrounding 
rocks  without  any  disturbing  element.  Fifty-eight 
measurements  at  equal  distances  from  the  surface  to 
the  greatest  depths  were  thus  obtained. 

We  have  now  to  discuss  the  instructive  results  to 
which .  we  have  been  conducted  by  this  remarkable 
series  of  measurements.  First  let  us  notice  that  there 
is  much  less  variation  in  the  subterranean  temperatures 
than  in  the  temperatures  on  the  earth's  surface.  On 
the  surface  of  the  earth  we  are  accustomed  to  large 
fluctuations  of  temperature.  We  have,  of  course,  the 
diurnal  fluctuations  in  temperature  from  day  to  night ; 
we  have  also  the  great  seasonal  fluctuations  between 
summer  and  winter.  But  below  a  certain  depth  in  the 
ground  the  temperature  becomes  much  more  equable. 
Whether  the  temperature  on  the  surface  be  high  or 
whether  it  be  low,  the  temperature  of  any  particular 
point  far  beneath  the  surface  does  not  change  to  any 
appreciable  extent.  In  Arctic  regions  the  surface  of 
the  earth  may  undergo  violent  seasonal  changes  of 
temperature,  while  at  a  few  feet  below  the  surface  the 
temperature,  from  one  end  of  the  year  to  the  other, 
may  remain  sensibly  unaltered. 

In  deep  and  extensive  caverns  the  temperature  is 
sometimes  found  to  remain  practically  unaffected  by 


THE  TEMPERATURE  BENEATH  THE  EARTH.    133 

the  changes  in  the  seasons.  The  Mammoth  Cave  of 
Kentucky  is  a  notable  instance.  The  uniformity  of 
the  temperature,  as  well  as  the  mildness  and  dryness 
of  the  air,  in  those  wonderful  subterranean  vaults  is 
such  that  many  years  ago  a  project  was  formed  to 
utilise  the  cavern  as  an  abode  for  consumptive  patients, 
for  whose  cure,  according  to  the  belief  then  prevailing, 
an  equable  temperature  was  above  all  things  to  be 
desired.  Houses  were  indeed  actually  built  on  the 
sandy  floors  of  the  cavern,  and  I  believe  they  were 
for  some  time  tenanted  by  consumptive  patients  willing 
to  try  this  desperate  remedy.  The  temperature  may 
have  been  uniform  and  the  air  may  have  been  dry, 
but  the  intolerable  gloom  of  such  a  residence  entirely 
neutralised  any  beneficial  effects  that  might  otherwise 
have  accrued.  The  ruins  of  the  houses  still  remain 
to  testify  to  the  failure  of  the  experiment. 

The  heat  received  from  the  sun  does  not  pene- 
trate far  into  the  earth's  crust,  and  consequently  the 
diurnal  and  even  the  seasonal  changes  of  the  tempera- 
ture at  the  surface  produce  less  and  less  effect  with 
every  increase  of  the  depth.  All  such  variations  of 
temperature  are  confined  to  within  100  feet  of  the 
surface.  At  the  depth  of  about  100  feet  a  fixed 
temperature  of  52°  Fahrenheit  is  reached,  and  this 
is  true  all  over  the  earth.  It  matters  not  whether 
the  surface  be  hot  or  cold,  whether  the  latitude 
is  tropical  and  the  season  is  midsummer,  whether 
the  latitude  lie  in  the  Arctic  regions  and  the  season 
be  the  awful  winter  of  iron-bound  frost  and  total 
absence  of  sun — in  all  cases  we  find  that  about 
100  feet  below  the  surface  the  temperature  is  52°. 
With  sufficient  accuracy  we  may  say  that  this 
10 


134  THE    EARTH'S    BEGINNING. 

depth  expresses  the  limit  of  the  penetration  of 
the  earth's  crust  by  sunbeams.  The  remarkable  law 
according  to  which  the  temperature  changes  below 
the  depth  of  100  feet  is  wholly  irrespective  of  the 
solar  radiation. 

The  study  of  the  internal  heat  of  the  earth  may 
be  said  to  begin  below  the  level  of  100  feet,  and  the 
results  that  were  obtained  in  the  great  boring  are 
extremely  accordant.  The  deeper  the  hole,  the  hotter 
the  rocks ;  and  Captain  Huyssen  found  that  for  each 
sixty-six  feet  in  descent  the  temperature  increased 
one  degree  Fahrenheit.  To  illustrate  the  actual  obser- 
vations, let  us  take  two  particular  cases.  We  have 
said  that  the  hole  was  one  mile  and  117  yards 
deep.  Let  us  first  suppose  the  thermometer  to  be 
lowered  117  yards  and  then  raised,  after  a  due 
observance  of  the  precautions  required  to  obtain  an 
accurate  result.  The  temperature  of  the  rocks  at  the 
depth  of  117  yards  is  thus  ascertained.  In  the  next 
observation  let  the  thermometer  be  lowered  from  the 
surface  to  the  bottom  of  the  hole,  that  is  to  say, 
exactly  one  mile  below  the  position  which  it  occupied 
in  the  former  experiment.  The  observations  indicate 
a  temperature  80°  Fahrenheit  higher  in  the  latter 
case  than  in  the  former.  We  have  thus  ascertained 
a  most  important  fact.  We  have  shown  that  the  tem- 
perature of  the  crust  of  the  earth  at  the  depth  of  one 
mile  increases  about  80°.  This  is  at  the  rate  of  one 
degree  every  sixty-six  feet.  I  should  just  add,  as  a 
caution,  that  if  we  choose  to  say  the  temperature 
increases  one  degree  per  sixty-six  feet  of  descent,  we 
ought  to  suppose  that  we  start  from  a  point  which 
is  not  higher  than  that  level  of  100  feet  above  which, 


THE  DEEPER  THE  HO  LE,  THE  ORE  A  TER  THE  HE  A  T.  135 

as  already  explained,  the  temperature  of  the  rocks 
is  more  or  less  affected  by  solar  heat. 

We  have  described  these  particular  observations  in 
some  detail  because  they  have  been  conducted  under 
conditions  far  more  favourable  to  accuracy  than  have 
ever  been  available  in  any  previous  investigations  of 
the  same  kind.  But  now  we  shall  omit  further  refer- 
ence to  this  particular  undertaking  near  Leipzig.  It 
is  not  alone  in  that  particular  locality,  not  alone  in 
Germany,  not  alone  in  Europe,  not  alone  on  the  surface 
of  any  continent,  that  this  statement  may  be  made. 
The  statement  is  one  universally  true  so  far  as  our 
whole  earth  is  concerned.  Wherever  we  bore  a  hole 
through  the  earth's  crust,  whether  that  hole  be  made 
in  the  desert  of  Sahara  or  through  the  icebound  coasts 
of  Greenland,  we  should  find  the  general  rule  to  obtain, 
that  there  is  an  increase  of  temperature  of  about  80°  for 
a  mile  of  descent.  This  is  true  in  every  continent,  it 
is  true  in  every  island ;  and,  though  we  cannot  here 
go  into  the  evidence  fully,  there  is  not  the  least  doubt 
that  it  is  true  also  under  the  floor  of  ocean.  If  beneath 
the  bed  of  the  Atlantic  a  hole  a  mile  deep  were  pierced, 
the  temperature  of  the  rocks  at  the  bottom  of  that 
hole  would,  it  is  believed,  exceed  by  about  80°  the 
temperature  of  the  rocks  at  the  surface  where  the 
hole  had  its  origin.  We  learn  that  at  the  depth  of  a 
mile  the  temperature  of  the  earth  must  generally  be 
80°  hotter  than  it  is  at  the  level  of  constant  tem- 
perature near  the  surface. 

It  may  perhaps  help  us  to  realise  the  significance 
of  this  statement  if  we  think  of  the  following  illustra- 
tion. Let  us  imagine  that  the  waters  of  the  ocean  were 
removed  from  the  earth.  The  ocean  may  in  places  be 


136  THE   EARTH'S    BEGINNING. 

five  or  six  miles  deep,  but  that  is  quite  an  inconsider- 
able quantity  when  compared  with  the  diameter  of 
the  earth.  The  change  in  the  size  of  the  earth  by  the 
removal  of  all  the  water  would  not  be  greater,  propor- 
tionally, than  the  change  produced  in  a  wet  football 
by  simply  wiping  it  dry.  Let  us  suppose  that  an 
outer  layer  of  the  earth's  surface,  a  mile  in  thickness, 
was  then  to  be  peeled  off.  If  we  remember  that  the 
diameter  of  the  earth  is  8,000  miles,  we  shall  see  that 
this  outer  layer,  whose  removal  we  have  supposed, 
does  not  bear  to  the  whole  extent  of  the  earth  a  ratio 
even  as  great  as  that  which  the  skin  of  a  peach  does 
to  the  fruit  inside.  But  this  much  is  certain,  that  if 
the  earth  were  so  peeled  there  would  be  a  wonderful 
difference  in  its  nature.  For  though  practically  of 
the  same  size  as  it  is  at  present,  it  would  be  so  hot 
that  it  would  be  impossible  to  live  upon  it. 

Next  comes  the  very  interesting  question  as  to 
the  temperature  that  would  be  found  at  the  bottom  of 
a  hole  deeper  still  than  that  we  have  been  consider- 
ing. Our  curiosity  as  to  the  depths  of  the  earth 
extends  much  below  the  point  to  which  Captain 
Huyssen  drove  down  his  diamond  drill.  The  trouble 
and  the  cost  of  still  deeper  exploration  of  the  same 
kind  seem,  however,  to  be  actually  prohibitive.  To 
bore  a  hole  two  miles  deep  would  certainly  cost  a 
great  deal  more  than  twice  the  sum  which  sufficed  to 
bore  a  hole  one  mile  deep.  At  a  great  depth  each 
further  foot  could  only  be  won  with  not  less  difficulty 
and  expense  than  a  dozen,  or  many  dozen  feet,  at 
the  surface.  Mining  enterprise  does  not  at  present 
seem  to  contemplate  actual  workings  at  depths  much 
over  a  mile,  so  there  does  not  seem  much  chance  of 


A    UNIFORM   INCREASE.  137 

any  very  much  deeper  boring  being  attempted.  We  do 
not  say  that  a  hole  two  miles  deep  would  be  actually 
impossible;  it  may  well  be  wished  that  some  million- 
aire could  be  induced  to  try  the  experiment.  We 
should  greatly  like  to  be  able  to  lower  a  thermometer 
down  to  a  depth  of  two  miles  through  the  earth's 
crust. 

Seeing  there  is  but  little  chance  of  our  wish  for 
such  future  experiments  being  gratified,  it  is  consola- 
tory to  find  that  actual  observations  of  this  kind  are 
not  indispensable  to  the  argument  on  which  we  are 
to  enter.  Our  argument  can  indeed  be  conducted  a 
stage  further,  even  with  our  present  information.  The 
indications  already  obtained  in  the  hole  one  mile  deep 
go  a  long  way  towards  proving  what  the  temperature 
of  a  hole  still  deeper  would  be.  We  have  already 
remarked  that  it  was  part  of  Captain  Huyssen's 
scheme  to  obtain  careful  readings  of  his  thermometer 
at  intervals  of  100  feet  from  the  surface  to  the 
bottom  of  the  hole.  A  study  of  these  readings  shows 
that  the  increase  of  80°  in  a  mile  takes  place  uniformly 
at  the  rate  of  one  degree  for  each  sixty-six  feet  of  depth. 
As  the  temperature  increases  uniformly  from  the  surface 
down  to  the  lowest  point  which  our  thermometers  have 
reached,  it  would  be  unreasonable  to  suppose  that  the 
rate  of  increase  would  be  found  to  suffer  some  abrupt 
change  if  it  were  possible  to  go  a  little  deeper.  As  the 
temperature  rises  80°  in  the  first  mile,  and  as  the 
rate  of  increase  is  shown  by  the  observations  to  be 
quite  as  large  at  the  bottom  of  the  hole  as  it  is  at 
the  top,  we  certainly  shall  not  make  any  very  great 
mistake  if  we  venture  to  assume  that  in  the  second 
mile  the  temperature  would  also  increase  to  an  extent 


138  THE    EARTH'S    BEGINNING. 

which  will  not  be  far  from  80°.  This  inference 
from  the  observations  leads  to  the  remarkable  con- 
clusion that  at  a  depth  of  two  miles  the  temperature 
of  the  earth  must  be,  we  will  not  say  exactly,  but  at 
all  events  not  very  far  from,  160°  higher  than  at  the 
level  of  constant  temperature  about  100  feet  down. 

As  in  the  former  case,  we  need  not  confine  our- 
selves to  any  particular  locality  in  drawing  this 
conclusion.  The  arguments  apply  not  only  to  the 
rocks  underneath  Leipzig,  but  to  the  rocks  over 
every  part  of  the  globe,  whether  on  continents  or 
islands,  or  even  if  forming  the  base  of  an  ocean. 
No  one  denies  that  the  law  of  increase  in  tempera- 
ture with  the  depth  must  submit  to  some  variation 
in  accordance  with  local  circumstances.  In  essential 
features  it  may,  however,  be  conceded  that  the  law  is 
the  same  over  all  the  earth.  If  we  take  52°  to  be 
the  temperature  of  the  level  100  feet  down,  which 
limits  the  seasonal  variations,  and  if  we  add  that  at 
two  miles  further  down  the  temperature  is  somewhere 
about  160°  more,  we  come  to  the  conclusion  that 
at  a  depth  of  a  little  over  two  miles  the  temperature 
of  the  rocks  forming  the  earth's  crust  is  about  212° 
Fahrenheit.  Thus  we  draw  the  important  inference 
that  if,  the  oceans  having  been  removed,  we  were  then 
to  remove  from  the  earth's  surface  a  rind  two  miles 
thick — a  thickness  which,  it  is  to  be  observed,  is  only 
the  two-thousandth  part  of  the  earth's  radius — we 
should  transform  the  earth  into  a  globe  which,  while 
it  still  retained  appreciably  the  same  size,  would  have 
such  a  temperature  that  even  the  coolest  spot  were 
as  hot  as  boiling  water.  This  is  indeed  a  remarkable 
result. 


DEEPER    AND    DEEPER    STILL.  139 

And  now  that  we  have  gone  so  far,  it  is  impossible 
for  us  to  resist  making  a  further  attempt  to  determine 
what  the  temperature  of  the  earth's  crust  must  be 
if  we  could  send  a  thermometer  still  lower.  A  hole 
one  mile  deep  we  have  seen ;  I  do  not  think  we  can 
hope  to  see  a  hole  two  miles  deep,  but  still  it  may 
not  be  absolutely  impracticable  ;  but  a  hole  of  three 
or  more  miles  deep  we  may  safely  regard  as  transcend- 
ing present  possibilities  in  engineering  enterprise.  Are 
we  therefore  to  be  deprived  of  all  information  as  to 
the  condition  of  our  earth  at  depths  exceeding  those 
already  considered  ?  Fortunately  we  can  learn  some- 
thing. We  are  assisted  by  certain  laws  of  heat,  and, 
though  the  evidence  on  which  we  believe  those  laws' 
is  necessarily  limited  to  the  experience  of  Nature  as 
it  comes  within  our  observation,  yet  it  is  impossible  to 
refuse  assent  to  the  belief  that  the  same  laws  will 
regulate  the  transmission  of  heat  in  the  crust  of  the 
earth  two  miles,  three  miles,  or  many  miles  beneath 
our  feet. 

I  represent,  in  the  diagram  shown  in  Fig.  23,  three 
consecutive  beds  of  rock — A,  B,  and  C — as  they  lie  in 
the  earth's  crust,  a  little  more  than  a  mile  beneath  our 
feet.  I  shall  suppose  that  the  bed  B  is  the  very 
lowest  rock  whose  temperature  was  determined  in  the 
great  boring.  The  drill  has  passed  completely  through 
A,  it  has  pierced  to  the  middle  of  B,  but  it  has  not 
entered  C.  The  observations  have  shown  that  the 
temperature  of  the  stratum  B  exceeds  that  of  the 
stratum  A,  and  we  further  note  that  this  is  a 
permanent  condition — that  is  to  say,  B  constantly 
remains  hotter  than  A.  From  this  fact  alone  we  can 
learn  something  as  regards  the  temperature  of  the 


140 


TEE    EARTH'S    BEGINNING. 


stratum  C  which  lies  in  contact  with  B.  Of  course 
we  are  unable  to  observe  the  temperature  of  C  directly, 
because  by  hypothesis  the  boring  tool  has  not  entered 
that  rock.  We  can,  however,  prove,  from  the  laws  of 
the  conduction  of  heat,  that  the  temperature  of  C 
must  be  greater  than  that  of  B ;  and  this  appears 

from    the    following 
consideration. 

It  is  plain  that 
C  must  be  either 
just  the  same  tem- 
perature as  B,  or  it 
must  be  hotter  than 
B,  or  it  must  be 
colder  than  B.  If  C 
were  the  same  tem- 
perature as  B,  then 
the  law  of  conduc- 
tion of  heat  tells  us  that  no  heat  would  flow 
from  one  of  these  strata  to  the  other.  The  laws 
of  heat,  however,  assure  us  that  when  two  bodies 
at  different  temperatures  are  in  contact  the  heat 
will  flow  from  the  hotter  of  these  bodies  into  the  colder, 
so  long  as  the  inequality  of  temperature  is  maintained, 
As  B  is  hotter  than  A,  then  heat  must  necessarily  flow 
from  B  into  A,  and  this  flow  must  tend  to  equalise  the 
temperature  in  these  strata,  for  B  is  losing  heat  while 
none  is  flowing  into  it  from  C.  Therefore  B  and  A 
could  not  continue  to  preserve  indefinitely  the  different 
temperatures  which  observation  shows  them  to  do.  We 
are  therefore  forced  to  the  conclusion  that  B  and  C 
cannot  be  at  the  same  temperature. 

Next  let  us  suppose   that   the  temperature   of   the 


23. — AT  THE  BOTTOM  or  THE 
GREAT  BORE. 


HOTTER    AND    HOTTER.  141 

stratum  B  exceeded  that  of  C.  Then,  as  A  is  colder 
than  B,  it  appears  that  B  would  be  lying  between  two 
strata  each  having  a  temperature  lower  than  itself.  But 
that,  of  course,  cannot  be  a  permanent  arrangement, 
for  the  heat  would  then  escape  from  B  on  both  sides. 
The  laws  of  heat,  therefore,  tell  us  that  B  could  not 
possibly  retain  permanently  a  temperature  above  both 
A  and  C.  Observation,  however,  shows  that  the  tem- 
peratures of  A  and  B  are  persistently  unequal.  We 
are  therefore  obliged  to  reject  the  supposition  that  the 
temperature  of  C  can  be  less  than  that  of  B. 

We  have  thus  demonstrated  that  the  temperature 
of  the  stratum  C  cannot  be  the  same  as  that  of  B. 
We  have  also  demonstrated  that  it  cannot  be  colder 
than  B.  We  must  therefore  believe  that  C  is  hotter 
than  B.  This  proves  that  the  stratum  immediately 
beneath  that  stratum  to  which  the  observations  have 
extended  must  be  hotter  than  it.  Thus,  though  the 
stratum  below  the  bottom  of  the  hole  lies  beyond  the 
reach  of  our  actual  observation,  we  have,  nevertheless, 
been  able  to  learn  something  with  regard  to  its 
temperature. 

Having  established  this  much,  we  can  continue  the 
same  argument  further ;  indeed,  it  would  seem  that  we 
can  continue  it  indefinitely,  so  long  as  there  is  a 
succession  of  such  strata.  Underneath  the  stratum  C 
must  lie  another  stratum  D.  But  we  have  shown  that 
C  must  be  hotter  than  B,  and  precisely  the  same  argu- 
ment that  has  proved  this  will  prove  that  D  is  hotter 
than  C.  Underneath  D  comes  the  stratum  E,  and 
again  the  same  argument  will  apply.  Inasmuch  as  D 
is  hotter  than  C,  it  follows  that  E  must  be  hotter  than 
D.  These  three  strata,  C,  D,  and  E,  are  all  beyond  the 


142  THE    EARTH'S    BEGINNING. 

reuch  of  the  thermometer,  we  know  nothing  of  their 
temperatures  by  direct  observation ;  but  none  the  less 
is  the  argument,  which  we  are  following  strictly,  ap- 
plicable. Thus  we  obtain  the  important  result  that 
in  the  crust  of  the  earth  the  temperature  must  be 
always  greater,  the  greater  the  depth  beneath  the 
surface. 

We  have  seen  that  the  rate  of  increase  of  tempera- 
ture with  the  depth  is  about  80°  for  the  first  mile,  and 
we  deem  it  probable  that  the  rate  of  increase  may  be 
maintained  at  about  the  same  for  the  second  mile. 
But  we  do  not  suppose  that  the  rate  of  increase  mile 
after  mile  will  remain  the  same  at  extremely  great 
depths.  It  may  perhaps  be  presumed  that  there  must 
be  some  increase  of  temperature  all  the  way  to  the 
earth's  centre ;  but  the  rate  of  increase  per  mile  may 
change  as  the  centre  is  approached.  The  point  of  im- 
portance for  our  present  argument  is,  that  the  tempera- 
ture of  the  earth  must  increase  with  the  depth,  though 
the  rate  of  increase  is  quite  unknown  to  us  at  depths 
greatly  beyond  those  which  the  thermometer  has 
reached.  It  is  easy  to  see  that  the  conditions  pre- 
vailing in  the  earth's  interior  might  greatly  modify 
any  conclusion  we  should  draw  from  observations  near 
the  surface.  Our  argument  has  been  based  on  the 
laws  of  heat,  as  we  find  them  existing  in  matter  on 
the  surface  of  the  earth  submitted  to  such  ranges  of 
different  physical  conditions  as  can  be  dealt  with  in 
our  laboratories ;  but  at  such  excessively  high  tempera- 
tures as  may  exist  in  the  earth's  interior  the  properties 
ol  matter  may  be  widely  different  from  the  properties 
of  matter  as  known  to  us  within  the  temperatures  that 
we  are  able  to  produce  and  control.  The  enormous 


THE  PRESSURE  IN  THE  EARTH'S  INTERIOR.  143 

pressure  to  which  matter  in  the  interior  of  the  e&rth 
must  be  subjected  should  also  be  mentioned  in  this 
connection.  It  is  wholly  impossible  to  produce  pres- 
sures by  any  mechanical  artifice  which  even  distantly 
approach  in  intensity  to  that  awful  force  to  which 
matter  is  subjected  in  the  earth's  interior. 

It  may  be  instructive  to  consider  a  few  facts  with 
respect  to  this  question  of  pressure  in  the  earth's 
interior.  A  column  of  water  thirty  feet  high  gives,  as 
everybody  knows,  a  pressure  of  fifteen  pounds  on  the 
square  inch.  It  will  be  quite  accurate  enough  for  our 
present  purpose  to  assume  that  the  average  density 
of  rock  is  three  times  that  of  water :  the  pressure  of 
ten  feet  of  rock  would  therefore  produce  the  same 
pressure  as  thirty  feet  of  water,  that  is  to  say,  fifteen 
pounds  on  the  square  inch.  The  pressure  due  to  the 
superincumbent  weight  of  a  mile  of  rock  would  be 
more  than  three  tons  on  the  square  inch.  At  the 
depth  of  ten  miles  beneath  the  earth's  surface  the 
pressure,  amounting  as  it  does  to  over  thirty  tons  on 
the  square  inch,  would  very  nearly  equal  the  pressure 
produced  on  the  inside  of  a  100- ton  gun  when  the 
charge  of  cordite  has  been  exploded  to  drive  the 
missile  forth.  This  is  indeed  about  as  large  a  pres- 
sure as  can  well  be  dealt  with  artificially,  for  we 
know  that  the  100-ton  gun  has  to  be  enormously 
strong  if  it  is  to  resist  this  pressure.  But  ten  miles 
of  rock  is  as  nothing  compared  with  the  thickness  of 
rock  that  produces  the  pressures  in  the  earth's  interior. 
Even  if  a  shell  of  rocks  ten  miles  thick  were  removed 
from  the  surface  it  would  alter  the  diameter  of  our 
globe  by  no  more  than  one  four-hundredth  part.  At 
the  depth  of  about  thirty  miles  from  the  surface  the 


144  TEE   EARTH'S   BEGINNING. 

pressure  in  the  earth's  interior  would  amount  to  some 
100  tons  on  each  square  inch.  With  each  increase  in 
depth  the  pressure  increases  enormously,  though  it 
may  not  be  correct  to  say  that  the  pressure  is  pro- 
portional to  the  depth.  A  pressure  of  1,000  tons  on 
the  square  inch  must  exist  at  a  depth  which  is 
still  quite  small  in  comparison  with  the  radius  of  the 
earth. 

We  .  have  not,  and  apparently  cannot  have,  the 
least  experimental  knowledge  of  the  properties  of 
matter  at  the  moment  when  it  is  subjected  to  pres- 
sure amounting  to  thousands  of  tons  per  square 
inch;  still  less  can  we  determine  the  behaviour  of 
matter  at  that  pressure  of  scores  of  thousands  of 
tons,  to  which  much  of  the  interior  of  the  earth  is 
at  this  moment  subjected.  Professor  Dewar,  in  his 
memorable  researches,  has  revealed  to  us  the  remark- 
able changes  exhibited  in  the  properties  of  matter 
when  that  matter  has  been  cooled  to  a  temperature 
which  lies  in  the  vicinity  of  absolute  zero.  We  can, 
however,  hardly  hope  that  any  experiments  will  give 
us  information  as  to  the  properties  of  matter  when 
heated  to  a  temperature  vastly  transcending  that 
which  could  ever  be  produced  in  our  most  powerful 
electric  furnaces,  and  at  the  same  time  exposed  to 
a  pressure  hundreds  of  times,  or  indeed  we  may  say 
thousands  of  times,  greater  than  any  pressure  that 
has  ever  been  produced  artificially  by  the  action  of 
the  most  violent  explosive  with  which  the  discoveries 
of  chemistry  have  made  us  acquainted. 

We  really  do  not  know  how  far  the  laws  of  heat, 
which  have  been  employed  in  showing  that  the  tem- 
perature must  increase  as  the  depth  increases,  can  be 


DIFFERENT    CONDITIONS. 


145 


Fig.  24. — THREE  CONSECUTIVE  SHELLS  OF  THE  EARTH'S  CRUST. 

considered  as  valid  under  the  extreme  condition  to 
which  matter  is  subjected  in  the  deep  interior  of  our 
globe.  The  laws  may  be  profoundly  modified.  It 
suffices,  fortunately  for  our  present  argument,  to  say 
that,  so  far  as  observations  have  been  possible,  the 
temperature  does  gradually  increase  with  the  depth, 
and  that  this  increase  must  still  continue  from 
stratum  to  stratum  as  greater  depths  are  reached, 
unless  it  should  be  found  that  by  the  excessive 
exaltation  of  temperature  and  the  vast  intensity  of 


146  THE    EARTH'S   BEGINNING. 

pressure  certain  properties  of  matter  become  so  trans- 
formed as  to  render  the  laws  of  heat,  as  we  know 
them,  inapplicable. 

In  subsequent  chapters  we  shall  have  some  further 
points  to  consider  with  respect  to  the  interior  of  the 
earth  and  its  physical  characteristics,  which  are,  how- 
ever, not  necessary  for  our  present  argument.  What 
we  now  desire  to  prove  can  be  deduced  from 
the  demonstrated  fact  that  the  earth's  temperature 
does  steadily  increase  from  the  level  of  constant 
temperature,  100  feet  below  the  surface,  down  to 
the  greatest  depth  to  which  thermometers  have  ever 
been  lowered.  We  may  presume  that  the  same  law 
holds  at  very  much  greater  depths,  even  if  it  does 
not  hold  all  the  way  to  the  centre. 

To  make  our  argument  clear,  let  us  think  of  three 
different  strata  of  rock.  This  time,  however,  we  shall 
suppose  them  to  cover  the  whole  earth,  and  we  shall 
consider  them  to  lie  within  the  first  mile  from  the 
surface;  they  will  thus  be  well  within  the  region  ex- 
plored by  observation  (Fig.  24).  We  shall  also  regard 
them  as  shells  of  uniform  thickness,  and  it  will  be  con- 
venient to  think  of  them  as  being  so  very  thin  that  we 
may  consider  any  one  of  the  shells  called  A  to  have 
practically  a  uniform  temperature.  The  next  shell  B 
immediately  inside  A  will  have  a  slightly  greater  tem- 
perature, and  be  also  regarded  as  uniform,  and  the 
shell  immediately  inside  that  again  will  have  a  tem- 
perature greater  still.  We  shall  call  the  innermost  of 
the  three  shells  C,  and  C  is  hotter  than  the  next  outer 
shell  B,  while  B  is  hotter  than  A.  The  laws  of  heat 
tell  us  that  as  B  and  A  are  in  contact,  and  that  as 
B  is  continually  hotter  than  A,  then  B  must  be  con- 


THE   FLOW   OF   BEAT.  147 

tinuously  transmitting  heat  to  A.  In  fact,  B  appears 
to  be  constantly  endeavouring  to  reduce  itself  to  the 
temperature  of  A  by  sharing  with  A  the  excess  of 
temperature  which  it  possesses.  But  if  we  consider 
the  relation  between  the  shell  B  and  the  hotter  shell 
C,  immediately  beneath  it,  we  see  that  precisely  the 
same  argument  will  show  that  B  is  constantly  receiv- 
ing heat  from  C.  We  thus  see  that  while  B  is  con- 
tinuously discharging  heat  from  its  outside  surface,  it 
is  as  constantly  receiving  heat  which  enters  through 
its  inside  surface.  Heat  enters  B  from  C,  and  heat 
passes  from  B  into  A,  so  that  B  is  in  fact  a  channel 
through  which  heat  passes  from  C  into  A. 

That  which  we  have  shown  to  take  place  in  these 
three  consecutive  layers  in  the  earth's  crust  must 
also  take  place  in  every  three  consecutive  layers. 
Each  layer  is  continually  receiving  heat  from  the 
layer  below,  and  is  as  constantly  communicating  heat 
to  the  layer  above.  No  doubt  the  rocks  are  very 
bad  conductors  of  heat,  so  that  the  transmission  of 
heat  from  layer  to  layer  is  a  very  slow  process.  But 
even  if  this  flow  of  heat  be  slow,  it  is  incessant,  so 
that  in  the  course  of  ages  large  quantities  of  heat  are 
gradually  transmitted  from  the  earth's  interior,  and 
ultimately  reach  the  level  of  constant  temperature. 
There  is  nothing,  however,  to  impede  their  outward 
progress,  so  at  last  the  heat  reaches  the  earth's 
surface. 

When  the  surface  has  been  reached,  then  another 
law  of  heat  declares  what  must  happen  next.  It  is, 
of  course,  by  conduction  that  the  heat  passes  from 
layer  to  layer  in  its  outward  progress,  until  it  ulti- 
mately gains  the  surface.  At  the  surface  the  heat  is 


148  THE    EARTH'S    BEGINNING. 

then  absolutely  removed  from  the  solid  earth  either 
by  the  convection  through  the  air  or  by  direct  radia- 
tion into  space. 

I  may  here  interrupt  the  argument  for  a  moment 
to  make  quite  clear  a  point  which  might  perhaps 
otherwise  offer  some  difficulty  to  the  reader.  When 
this  outward  flow  of  heat  reaches  the  superficial 
layers  it  becomes,  of  course,  mixed  up  with  the 
heat  which  has  been  absorbed  by  the  soil  from 
the  direct  radiation  of  the  sun,  and  thig  varies,  of 
course,  with  the  hour  of  the  day  and  with  the  season 
of  the  year.  The  heat  which  steadily  leaks  from  the 
interior  has  an  effect  on  the  rocks  near  the  surface, 
Avhich  is  only  infinitesimal  in  comparison  with  the 
heat  which  they  receive  from  periodic  causes.  We 
may,  however,  say  that  whatever  would  be  the  tem- 
perature of  the  rock,  so  far  as  the  periodic  causes  are 
concerned,  the  actual  temperature  is  always  to  some 
minute  extent  increased  by  reason  of  the  heat  from 
the  earth's  interior.  The  argument  is,  perhaps,  still 
clearer  if,  instead  of  attending  to  the  earth's  sur- 
face, we  think  only  of  that  shell,  some  100  feet 
down,  which  marks  the  limit  of  the  depth  to  which 
the  seasonal  and  diurnal  variations  of  heat  extend. 
The  argument  shows  how  the  internal  heat  of  the 
earth,  passing  from  shell  to  shell  in  the  interior, 
reaches  this  layer  of  constant  temperature,  and  pass- 
ing through  it,  enters  into  those  superficial  strata  of 
the  earth  which  are  exposed  to  the  seasonal  varia- 
tions. With  what  befalls  that  heat  ultimately  we 
need  not  now  concern  ourselves ;  it  suffices  for  our 
argument  to  show  that  there  is  a  current  of  heat  out- 
ward across  this  level.  It  is  a  current  which  is  never 


THE  LEAKAGE  OF  HEAT  FROM  THE  EARTH.    149 

reversed,  and  consequently  must  produce  a  never- 
ceasing  drainage  from  the  heat  with  which  it  would 
seem  that  the  interior  of  the  earth  is  so  copiously 
provided. 

Calculations  have  been  made  to  ascertain  how 
much  heat  passes  annually  from  the  earth's  interior, 
across  this  surface  of  constant  temperature,  out  into 
the  superficial  regions  from  which  in  due  course  it 
becomes  lost  by  radiation.  A  convenient  way  of  measur- 
ing a  quantity  of  heat  is  by  the  amount  of  ice  it 
will  melt,  for  of  course  a  definite  quantity  of  heat  is 
required  to  melt  a  definite  quantity  of  ice.  It  has 
been  estimated  by  Professor  J.  D.  Everett,  F.R.S.,  that 
the  amount  of  internal  heat  escaping  from  our  earth 
each  year  would  be  sufficient  to  melt  a  shell  of  ice 
one-fifth  of  an  inch  thick  over  the  whole  surface  of 
the  globe.  We  cannot  indeed  pretend  that  any  de- 
termination of  the  actual  loss  of  heat  which  our  earth 
experiences  could  be  very  precise.  Sufficient  obser- 
vations have  not  yet  been  obtained,  for  the  operation 
is  so  slow  that  an  immense  period  would  have  to 
elapse  before  the  total  quantity  of  heat  lost  would 
be  sufficient  to  produce  effects  large  enough  to  be 
measured  accurately.  But  now  let  us  hasten  to  add 
that,  for  the  argument  as  to  the  nebular  theory  with 
which  we  are  at  present  concerned,  it  is  not  really 
material  to  know  the  precise  rate  at  which  heat  is 
lost.  It  is  absolutely  certain  that  a  perennial  leakage 
of  heat  from  the  interior  of  the  earth  does  take  place. 
This  fact,  and  not  the  amount  of  that  leakage,  is  the 
essential  point. 

And  this  loss,  which  is  at  present  going  on,  has 
been  going  on  continually.  Heat  from  the  earth  has 
11 


150  THE    EARTH'S    BEGINNING. 

been  lost  this  year  and  last  year;  it  has  been  lost 
for  hundreds  of  years  and  for  thousands  of  years. 
Not  alone  during  the  periods  of  human  history  has 
the  earth's  heat  been  declining.  Even  throughout 
those  periods,  those  overwhelming  periods  which 
geology  has  revealed  to  us,  has  this  earth  of  ours 
been  slowly  parting  with  its  heat. 

Let  us  pursue  this  reflection  to  its  legitimate  con- 
sequence. Whatever  may  ultimately  become  of  that 
heat,  it  is  certain  that  once  radiated  into  space  it  is 
lost  for  ever  so  far  as  this  globe  is  concerned.  You 
must  not  imagine  that  the  warm  beams  of  the  sun 
possess  any  power  of  replenishment  by  which  they 
can  restore  to  the  earth  the  heat  which  it  has 
been  squandering  for  unlimited  ages;  we  have  already 
explained  that  the  effect  of  the  heat  radiated  to  us 
from  the  sun  is  purely  superficial.  Even  amid  the 
glories  of  the  tropics,  even  in  the  burning  heat  of 
the  desert,  the  vertical  sun  produces  no  appreciable 
effects  at  depths  greater  than  this  critical  limit, 
which  is  about  100  feet  below  the  surface.  The 
rigours  of  an  Arctic  winter  have  as  little  effect  in 
reducing  the  temperature  of  the  rocks  at  that  depth 
as  the  torrid  heat  at  the  Equator  has  in  raising  it 
The  effect  in  each  case  is  nothing. 

The  argument  which  we  are  here  employing  to 
deduce  the  nebulous  origin  of  our  earth  from  the 
increase  of  temperature  with  increase  in  depth  in  the 
earth's  crust  must  be  cleared  from  an  objection.  It 
is  necessary  to  explain  the  matter  fully,  because  it 
touches  on  a  doctrine  of  very  great  interest  and  im- 
portance. 

That   a   rotating    body   should    possess   a   quantity 


THE    EARTH-MOON   SYSTEM.  151 

of  energy  in  virtue  of  its  rotation  will  be  familiar  to 
anyone  who  has  ever  turned  a  grindstone  or  watched 
the  fly-wheel  of  an  engine.  A  certain  amount  of 
work  has  to  be  expended  to  set  the  heavy  wheel 
into  rotation,  and  when  the  machine  is  called  upon 
to  do  work  it  will  yield  up  energy  and  its  motion 
will  undergo  a  corresponding  abatement.  The  heavy 
fly-wheel  of  the  machine  in  a  rolling  mill  contains,  in 
virtue  of  its  motion,  enough  energy  to  overcome  the 
tremendous  resistance  of  the  materials  submitted  to 
it.  Once  upon  a  time  the  earth  revolved  upon  its 
axis  in  six  hours,  instead  of  in  the  twenty-four  hours 
which  it  now  requires.  At  that  time  the  energy  of 
the  rotation  must  have  been  sixteenfold  what  it  is  at 
present.  This  consideration  shows  that  fifteen-six- 
teenths of  the  energy  that  the  earth  originally  pos- 
sessed in  its  rotation  has  disappeared,  and  we  want 
to  know  what  has  become  of  it. 

We  are  here  entering  upon  a  matter  of  some  dif- 
ficulty. It  is  connected  with  that  remarkable  chapter 
in  astronomy  which  describes  the  evolution  of  the 
earth-moon  system.  The  moon  was  originally  a  part 
of  ftie  earth,  for  in  very  early  times,  when  the  earth 
was  still  in  a  plastic  state,  a  separation  would  seem 
to  have  taken  place,  by  which  a  small  piece  broke 
off  to  form  the  moon,  which  has  been  gradually 
revolving  in  an  enlarging  orbit  until  it  has  attained 
the  position  it  now  occupies.  A  considerable  portion 
of  the  energy  of  the  earth's  rotation  has  been  applied 
to  the  purpose  of  driving  the  moon  out  to  its  present 
path,  but  there  is  a  large  remainder  which  cannot  be 
so  accounted  for.  It  is  well  known  that  the  evolution 
of  the  moon  has  been  a  remarkable  consequence  of 


152  THE    EARTH'S    BEGINNING. 

tidal  action.  There  are  tides  which  sway  to  and 
fro  in  the  waters  on  the  earth's  surface;  there  are 
tides  in  any  molten  or  viscous  matter  that  the  earth 
may  contain,  and  there  are  even  certain  small  tidal 
displacements  in  the  solid  material  of  our  globe. 
Tides  of  any  kind  will  generate  friction,  and  friction 
produces  heat,  and  the  energy  of  the  earth's  rotation, 
which  we  have  not  been  able  to  account  for  other- 
wise, has  been  thus  transformed  into  heat.  Through- 
out the  whole  interior  of  the  earth  heat  has  been 
produced  by  the  tidal  displacement  of  its  parts.  The 
question  therefore  arises  as  to  whether  the  internal 
heat  of  the  earth  may  not  receive  an  adequate  ex- 
planation from  this  tidal  action,  which  is  certainly 
sufficient  as  to  quantity.  It  is  easy  to  calculate  what 
the  total  quantity  of  this  tidal  heat  may  have  been. 
We  know  the  energy  which  the  earth  had  when  it 
rotated  in  six  hours,  and  we  know  that  it  now  retains 
no  more  than  a  sixteenth  of  that  amount.  We 
know  also  precisely  how  much  was  absorbed  in 
the  removal  of  the  moon,  and  the  balance  can  be 
evaluated  in  heat.  It  can  be  shown,  and  the  fact 
is  a  very  striking  one,  that  the  quantity  of  heat 
thus  arising  would  be  sufficient  to  account  many 
times  over  for  the  internal  heat  of  the  earth.  It 
might  therefore  be  urged  plausibly  that  the  internal 
heat  which  we  actually  find  has  had  its  origin  in 
this  way.  And  if  this  were  the  case  the  argument 
which  we  are  using  in  favour  of  the  nebular  origin 
of  the  earth,  would  be,  of  course,  invalidated. 

We  may  state  the  issue  in  a  slightly  different  manner, 
as  follows.  Heat  there  is  undoubtedly  in  the  earth; 
that  heat  might  have  come  from  the  primaeval  nebula 


TIDAL    FRICTION.  153 

as  we  have  supposed,  and  as  in  actual  fact  it  did 
come.  But  apparently  it  might  have  come  from  the 
tidal  friction.  Why  then  are  we  entitled  to  reject  the 
latter  view,  and  say  that  the  tidal  friction  will  not 
explain  the  internal  heat,  and  why  are  we  compelled 
to  fall  back  on  the  only  other  explanation  ? 

Lord  Kelvin  suggested  a  test  for  deciding  to  which 
of  these  two  sources  the  earth's  internal  heat  was  to 
be  attributed.  Professor  G.  H.  Darwin  applied  the 
test  and  decided  the  issue.  We  have  dwelt  upon  the 
rate  at  which  the  heat  increases  with  the  descent, 
this  rate  being  about  one  degree  every  sixty-six  feet. 
Now  the  distribution  of  the  heat,  if  it  had  come  from 
the  tidal  action,  would  be  quite  different  from  the  dis- 
tribution which  would  result  from  the  gradual  efflux 
of  heat  from  the  centre  in  the  process  of  cooling. 
And,  speaking  quite  generally,  we  may  surmise  that 
the  heat  produced  by  tidal  friction  would  be  distri- 
buted rather  more  towards  the  exterior  of  the  earth 
than  at  its  centre.  We  might  therefore  reasonably 
expect  that  if  the  internal  heat  of  the  earth  arose 
from  tidal  friction  it  would  be  more  uniformly  dis- 
tributed throughout  the  globe,  and  there  would  not 
be  so  great  a  contrast  between  the  high  temperature 
of  the  interior  and  the  lesser  temperatures  near  the 
surface  as  there  is  when  the  heat  distribution  is  merely 
the  result  of  cooling.  It  has  been  proved  that  if 
the  internal  heat  had  its  origin  from  the  tidal  friction, 
the  rate  of  increase  with  the  depth  would  be  totally 
different  from  what  it  is  actually  found  to  be.  It  would 
be  necessary  to  go  down  2,000  feet  to  obtain  an  in- 
crease of  one  degree,  instead  of  only  sixty-six  feet,  as 
is  actually  the  case. 


154  THE    EARTH'S    BEGINNING. 

Hence  we  conclude  that  the  increasing  heat  met 
with  in  descending  through  the  earth's  crust  is  not 
to  be  explained  by  tidal  friction ;  it  has  its  origin  in 
the  other  alternative,  namely,  from  the  cooling  of  the 
primaeval  nebula.  The  heat  which  was  undoubtedly 
produced  by  the  tidal  friction  has  gradually  become 
blended  with  the  heat  from  the  other,  and,  as  we 
must  now  say,  the  principal  source.  The  facts  with 
regard  .to  the  rate  of  increase  with  depth  thus  show 
that,  whatever  the  tides  may  have  done  in  producing 
internal  heat,  there  has  been  another  and  a  still  more 
potent  cause  in  operation.  The  important  conclusion 
for  our  present  purpose  is  that  our  argument  may 
justly  proceed  without  taking  account  of  the  effect 
of  tidal  friction. 

We  are  led  by  these  considerations  to  a  knowledge 
of  a  great  transformation  in  the  nature  of  our  globe 
which  must  have  occurred  in  the  course  of  ages.  We 
have  seen  that  this  earth  is  gradually  losing  heat  from 
its  interior,  and  we  have  seen  that  this  loss  of  heat 
is  incessant.  From  the  fountains  of  heat,  still  so 
copious,  in  the  interior  the  supply  is  gradually  dis- 
sipating. Now  heat  is  only  a  form  of  energy,  and 
energy,  like  matter,  cannot  itself  be  created  out  of 
nothing.  There  can  be  no  creation  of  heat  in  our 
earth  without  a  corresponding  expenditure  of  energy. 
If,  therefore,  the  earth  is  radiating  heat,  then,  as  there 
is  no  known  or,  indeed,  conceivable  source  of  energy 
by  which  an  equivalent  can  be  restored,  it  follows  that 
the  earth  must  have  less  internal  heat  now  than  it 
had  at  any  earlier  period.  No  doubt  the  process  of 
cooling  is  excessively  slow.  The  earth  has  less  internal 
heat  at  present  than  it  had  a  hundred  years  ago,  but 


THE    EARTH  AS    HOT   A8   SOILING    WATER.  155 

I  do  not  suppose  that  even  in  a  thousand  years,  or 
perhaps  in  ten  thousand  years,  there  would  be  any 
appreciable  decline  in  the  quantity  of  heat,  so  far  as 
any  obvious  manifestations  of  that  heat  are  concerned. 
It  is,  however,  certain  that  the  earth  must  have  been 
hotter,  even  though  there  are  not  any  observations 
to  which  we  can  appeal  to  verify  the  statement ;  and 
as  our  retrospect  extends  further  and  still  further 
through  the  ages  we  see  that  the  globe  must  have 
been  ever  hotter  and  ever  still  hotter.  Whatever  be 
the  heat  contained  in  our  earth  now,  it  must  have 
contained  vastly  more  heat  ten  million  years  ago ;  how 
otherwise  could  the  daily  leakage  of  heat  for  all  those 
ten  million  years  have  been  supplied  ?  It  follows  that 
there  must  have  been  much  more  heat  somewhere  in 
our  earth  ten  million  years  ago  than  there  is  at  present, 
and  the  further  our  retrospect  extends  the  hotter  do 
we  find  the  earth  to  have  been.  There  was  a  time 
when  the  temperature  of  the  earth's  surface  must  have 
been  warmed  not  alone  by  such  sunbeams  as  fell  upon 
it,  but  by  the  passage  of  the  heat  from  the  interior. 

No  matter  how  early  be  the  period  which  we 
consider,  we  find  the  same  causes  to  be  in  operation. 
There  was  a  time  when,  owing  to  the  internal  heat, 
the  surface  of  the  earth  must  have  been  as  hot  as 
boiling  water.  The  loss  of  heat  by  radiation  must 
then  have  taken  place  much  more  copiously  than  it 
does  at  present.  The  argument  we  are  pursuing  must 
therefore  have  applied  with  even  greater  force  in  those 
early  days.  There  was  a  time  when  the  materials  at 
the  surface  of  the  earth  must  have  been  intensely 
heated,  when  they  must  have  even  been  red  hot. 
There  was  a  time  when  the  earth's  surface  must  have 


156  THE    EARTH'S    BEGINNING. 

had  a  temperature  like  that  of  the  lava  as  it  issues 
from  a  volcano.  There  must  have  been  a  time  when 
the  surface  of  the  earth  was  not  even  solid,  when 
indeed  it  was  a  viscid  liquid,  and  earlier  still  the  liquid 
must  have  been  more  and  more  incandescent.  From 
that  brilliant  surface  heat  was  vehemently  radiated. 
Each  day  the  globe  was  hotter  than  on  the  succeeding 
day.  There  is  no  break  in  the  argument.  We  have 
to  think  of  this  glowing  globe  passing  through  those 
phases  through  which  we  know  that  all  matter  will 
pass  if  only  we  apply  to  it  sufficient  heat.  The  globe 
assumed  the  liquid  state  from  that  state  which  is 
demanded  by  a  temperature  still  higher,  the  state  in 
which  the  matter  is  actually  in  the  form  of  vapour. 
Even  the  most  refractory  substances  will  take  the 
form  of  vapour  at  a  very  high  temperature. 

Thus  we  are  conducted  to  a  remarkable  conception 
of  the  condition  in  which  the  materials  now  forming 
our  solid  earth  must  have  been  in  the  exceedingly 
remote  past.  What  is  now  our  earth  must  once  have 
been  a  great  quantity  of  heated  vapour.  It  need 
hardly  be  said  that  in  that  form  the  volume  of  the 
earth  was  much  larger  than  the  volume  which  the 
earth  has  at  present,  while  no  doubt  the  mass  of  the 
earth  then  was  even  less  than  the  mass  of  the  earth 
now,  by  reason  of  the  meteoric  matter  which  has 
been  drawn  in  by  our  globe. 

But  even  when  our  earth  was  in  this  inflated 
state  of  vapour  our  argument  can  be  still  maintained. 
Thus  we  see  that  the  earth,  or  rather  the  cloud  of 
vapour  which  was  ultimately  to  form  the  earth,  is  ever 
growing  larger  and  larger  in  our  retrospect,  ever  be- 
coming more  and  more  rarefied ;  and  it  may  well  have 


THE    EARTH   A    NEBULA.  157 

been  that  there  was  a  time  when  the  materials  of  this 
earth  occupied  a  volume  thousands  of  times  greater 
than  they  do  at  present. 

In  a  previous  chapter  we  have  seen  how  the  sun 
was  at  one  time  in  the  nebulous  state,  and  now  we 
have  been  led  to  a  similar  conclusion  with  regard  to 
the  earth.  At  that  time,  of  course,  the  sun  was  greatly 
in  excess  of  its  present  dimensions,  and  the  earth  was 
also  greatly  swollen.  The  nebula  which  formed  our 
sun,  and  the  nebula  which  formed  our  earth,  were  both 
so  vast  as  to  be  confluent;  they  were  indeed  both  part 
of  the  same  vast  nebula. 

Such  has  been  the  Earth's  Beginning  so  far  as 
modern  science  can  make  it  clear  to  us.  We  have 
at  least  indicated  the  course  which  events  must  have 
taken  according  to  the  laws  of  nature  as  we  under- 
stand them.  Many  of  the  details  of  the  great  evolution 
are  no  doubt  unknown  at  present,  and  perhaps  must 
ever  remain  so.  That  the  events  which  we  have 
endeavoured  to  describe  do  substantially  represent  the 
actual  evolution  of  our  system  is  the  famous  Nebular 
Theory. 


CHAPTER  IX. 

EARTHQUAKES  AND  VOLCANOES. 

Interior  of  the  Earth— Illustration  from  Norway— Solids  and  Liquids  — 
Rigidity  of  the  Interior  of  the  Earth— Earthquakes,  how  caused— 
Their  Testimony  as  to  the  Rigidity  of  the  Earth— Delicate  Instru- 
ment for  Measuring  Earthquake  Tremors— The  Seismometer- 
Professor  Milne's  Work  in  the  Isle  of  Wight— Different  Earth, 
quake  Groups— Precursors  and  Echoes — Vibrations  transmitted 
through  the  Earth's  Centre— Earthquakes  in  England— Other 
Evidence  of  the  Earth's  Rigidity— Krakatoa,  August  27th,  1883— 
The  Sounds  from  Krakatoa— The  Diverging  Waves— The  Krakatoa 
Dust — The  Hurricane  Overhead — Strange  Signs  in  the  Heavens — 
The  Blood-red  Skies. 

IN  this  chapter  we  shall  learn  what  we  can  as  to  the 
physical  condition  of  the  interior  of  our  earth  so  far 
as  it  may  be  reasonably  inferred  from  the  facts  of 
observation.  We  have  already  explained  in  the  last 
chapter  that  a  very  high  temperature  must  be  found 
at  the  depth  of  even  a  small  fraction  of  the  earth's 
radius,  and  we  have  pointed  out  that  the  excessively 
high  pressure  characteristic  of  the  earth's  interior 
must  be  borne  in  mind  in  any  consideration  as  to  the 
condition  of  the  matter  there  found. 

Let  us  take,  for  instance,  that  primary  question  in 
terrestrial  physics,  as  to  whether  the  interior  of  the 
earth  is  liquid  or  solid.  If  we  were  to  judge  merely 


18    THE   EARTH'S   INTERIOR  LIQUID?         159 

from  the  temperatures  reasonably  believed  to  exist  at 
a  depth  of  some  twenty  miles,  and  if  we  might  over- 
look the  question  of  pressure,  we  should  certainly  say 
that  the  earth's  interior  must  be  in  a  fluid  state.  It 
seems  at  least  certain  that  the  temperatures  to  be 
found  at  depths  of  two  score  miles,  and  still  more  at 
greater  depths,  must  be  so  high  that  the  most  refrac- 
tory solids,  whether  metals  or  minerals,  would  at  once 
yield  if  we  could  subject  them  to  such  temperatures 
in  our  laboratories.  At  such  temperatures  every  metal 
would  become  fluid,  even  if  it  were  not  transformed 
into  a  cloud  of  vapour.  But  none  of  our  laboratory 
experiments  can  tell  us  whether,  under  the  pressure 
of  thousands  of  tons  on  the  square  inch,  the  applica- 
tion of  any  heat  whatever  would  be  adequate  to  trans- 
form solids  into  liquids.  It  may  indeed  be  reasonably 
doubted  whether  the  terms  solids  and  liquids  are  appli- 
cable, in  the  sense  in  which  we  understand  them,  to 
the  materials  forming  the  interior  of  the  earth. 

It  was  my  good  fortune  some  years  ago  to  enjoy 
a  most  interesting  trip  to  Norway,  in  company  with 
a  distinguished  geologist.  Under  his  guidance  I  there 
saw  evidence  which  demonstrates  conclusively  that, 
when  subjected  to  great  pressure,  solids,  as  we  should 
call  them,  behave  in  a  manner  which,  if  not  that  of 
actual  liquids,  resembles  at  all  events  in  some  of  its 
characteristics  the  behaviour  of  liquids.  These  rocks 
in  some  places  are  conglomerates,  of  which  the  lead- 
ing constituents  are  water-worn  pebbles  of  granite. 
These  pebbles  are  of  various  sizes,  from  marbles  to 
paving-stones.  In  some  parts  of  the  country  these 
granite  pebbles  remain  in  the  form  which  they 
acquired  on  the  beach  on  which  they  were  rolled  by 


160  THE    EARTH'S    BEGINNING. 

the  primaeval  ocean;  in  other  parts  of  the  same 
interesting  region  the  form  of  the  pebbles  has  been 
greatly  changed  from  what  it  was  originally.  For  in 
the  course  of  geological  periods,  and  after  the  pebbles 
had  become  consolidated  into  the  conglomerate,  the 
rock  so  formed  had  been  in  some  cases  submitted  to 
enormous  pressure.  This  may  have  been  lateral  pres- 
sure, such  as  is  found  to  have  occurred  in  many  other 
places,  where  it  has  produced  the  well-known  geo- 
logical phenomenon  of  strata  crumpled  into  folds.  In 
the  present  case,  however,  it  seemed  more  probable 
that  it  was  the  actual  weight  of  the  superincumbent 
rocks,  which  once  lay  over  these  beds  of  conglomerate, 
which  produced  the  surprising  transformation.  It 
seems  to  be  not  at  all  improbable  that  at  one  time 
these  beds  of  conglomerate  must  have  been  covered 
with  strata  of  which  the  thickness  is  so  great  that  it 
may  actually  be  estimated  by  miles.  There  has,  how- 
ever, been  immense  denudation  of  the  superficial  rocks 
in  this  part,  at  all  events,  of  Norway,  so  that  in  the 
course  of  ages  these  strata,  overlying  the  conglomerate 
for  ages,  have  been  so  far  worn  away,  and  indeed 
removed,  by  the  action  of  ice  and  the  action  of  water 
that  the  conglomerate  is  now  exposed  to  view.  It 
offers  for  our  examination  striking  indications  of  the 
enormous  pressure  to  which  it  was  subjected  during 
the  incalculable  ages  of  geological  time. 

The  effect  of  this  long  continuance  of  great  pressure 
upon  the  pebbles  of  the  conglomerate  in  certain  parts 
of  the  country  has  been  most  astonishing.  The 
granite  in  the  pebbles  still  retains  its  characteristic 
crystalline  structure ;  it  has  obviously  not  undergone 
anything  that  could  be  described  as  fusion ;  yet  under 


A      VALLEY   IN  NORWAY.  161 

the  influence  of  the  two  factors  of  that  pressure,  namely, 
its  intensity  and  its  long  continuance,  the  granite 
pebbles  have  yielded.  In  some  cases  they  are  slightly 
elongated,  in  others  they  are  much  elongated,  while  in 
yet  others  they  are  even  rolled  out  flat.  At  different 
places  along  the  valley  the  various  phases  of  the  trans- 
formation can  be  studied.  We  can  find  places  where 
the  pebbles  seem  little  altered,  and  then  we  can  trace 
each  stage  until  the  solid  granite  pebbles  have,  by  the 
application  of  excessive  pressure,  been  compressed  into 
thin  sheets  whose  character  it  would  not  have  been 
easy  to  divine  if  it  had  not  been  possible  to  trace  out 
their  history.  These  sheets  lie  close  and  parallel,  so 
that  the  material  thus  produced  acquires  some  of  the 
characteristics  of  slate.  It  splits  easily  along  the 
flattened  sheets,  and  this  rolled-out  conglomerate  is 
indeed  actually  used  as  a  substitute  for  slate,  and  in 
some  places  there  are  houses  roofed  with  the  con- 
glomerate which  has  been  treated  in  this  extraordinary 
fashion. 

This  fact  will  illustrate  a  principle,  already  well 
known  in  the  arts,  that  many,  if  not  all,  solids  may 
be  made  to  flow  like  liquids  if  only  adequate  pressure 
be  applied.  The  making  of  lead  tubes  is  a  well- 
known  practical  illustration  of  the  same  principle,  for 
these  tubes  are  simply  formed  by  forcing  solid  lead  by 
the  hydraulic  press  through  a  mould  which  imparts 
the  desired  form. 

If  then  a  solid  can  be  made  to  behave  like  a 
liquid,  even  with  such  pressures  as  are  within  our 
control,  how  are  we  to  suppose  that  the  solids  would 
behave  with  such  pressures  as  those  to  which  they  are 
subjected  in  the  interior  of  the  earth?  The  fact  is 


162  THE   EARTH'S    BEGINNING. 

that  the  terms  solid  and  liquid,  at  least  as  we  under- 
stand them,  appear  to  have  no  physical  meaning  with 
regard  to  bodies  subjected  to  these  stupendous  pres- 
sures, and  this  must  be  carefully  borne  in  mind  when 
we  are  discussing  the  nature  of  the  interior  of  the 
earth. 

It  must,  however,  be  admitted  that  the  interior  of 
the  earth  in  its  actual  physical  state  seems  to  possess 
at  least  one  of  the  most  important  characteristics  of  a 
solid,  for  it  seems  to  be  intensely  rigid.  We  mean  by 
this,  that  the  material  of  the  earth,  or  rather  each 
particle  of  that  material,  is  very  little  inclined  to  move 
from  its  position  with  reference  to  the  adjacent  particles 
by  the  application  of  force.  Possibly  a  liquid,  such  as 
water,  might  not  behave  very  differently  in  this  respect 
from  a  solid  such  as  cast  iron,  if  each  of  them  were 
exposed  to  a  pressure  of  scores  of  thousands  of  tons 
per  square  inch,  as  are  the  materials  which  form  the 
great  bulk  of  the  earth.  But,  without  speculating  on 
these  points,  we  are  able  to  demonstrate  that  the  earth, 
as  a  whole,  does  exhibit  extreme  rigidity.  This  is  one 
of  the  most  remarkable  discoveries  which  has  ever  been 
made  with  regard  to  the  physics  of  our  earth.  The 
discovery  that  the  earth  is  so  rigid  is  mainly  due  to 
Lord  Kelvin. 

We  shall  now  mention  the  line  of  evidence  which 
appears  to  prove,  in.  the  simplest  and  most  direct 
manner,  the  excessive  rigidity  of  our  earth.  It  is  derived 
from  the  study  of  earthquake  phenomena,  and  we  must 
endeavour  to  set  it  forth  with  the  completeness  its 
importance  deserves. 

As  to  the  immediate  cause  01  earthquakes,  there  is 
no  doubt  considerable  difference  of  opinion.  But  I  think 


THE    CAUSE    OF   EARTHQUAKES.  163 

it  will  not  be  doubted  that  an  earthquake  is  one  of 
the  consequences,  though  perhaps  a  remote  one,  of  the 
gradual  loss  of  internal  heat  from  the  earth.  As  this 
terrestrial  heat  is  gradually  declining,  it  follows  from 
the  law  that  we  have  already  so  often  had  occasion  to 
use  that  the  bulk  of  the  earth  must  be  shrinking. 
No  doubt  the  diminution  in  the  earth's  diameter,  due 
to  the  loss  of  heat,  must  be  excessively  small,  even  in 
a  long  period  of  time.  The  cause,  however,  is  con- 
tinually in  operation,  and  accordingly  the  crust  of  the 
earth  has,  from  time  to  time,  to  be  accommodated  to 
the  fact  that  the  whole  globe  is  lessening.  The  cir- 
cumference of  our  earth  at  the  Equator  must  be 
gradually  declining ;  a  certain  length  in  that  circum- 
ference is  lost  each  year.  We  may  admit  that  loss 
to  be  a  quantity  far  too  small  to  be  measured  by  any 
observations  as  yet  obtainable,  but,  nevertheless,  it  is 
productive  of  phenomena  so  important  that  it  cannot 
be  overlooked. 

It  follows  from  these  considerations  that  the  rocks 
which  form  the  earth's  crust  over  the  surface  of  the 
continents  and  the  islands,  or  beneath  the  beds  of 
ocean,  must  have  a  lessening  acreage  year  by  year. 
These  rocks  must  therefore  submit  to  compression, 
either  continuously  or  from  time  to  time,  and  the 
necessary  yielding  of  the  rocks  will  in  general  take 
place  in  those  regions  where  the  materials  of  the 
earth's  crust  happen  to  have  comparatively  small 
powers  of  resistance.  The  acts  of  compression  will 
often,  and  perhaps  generally,  not  proceed  with  uni- 
formity, but  rather  with  small  successive  shifts,  and 
even  though  the  displacements  of  the  rocks  in  these 
shifts  be  actually  very  small,  yet  the  pressures  to 


164  THE    EARTH'S   BEGINNING. 

which  the  rocks  are  subjected  are  so  vast  that  a  very 
small  shift  may  correspond  to  a  very  great  terrestrial 
disturbance. 

Suppose,  for  instance,  that  there  is  a  slight  shift 
in  the  rocks  on  each  side  of  a  crack,  or  fault,  at  a 
depth  of  ten  miles.  It  must  be  remembered  that  the 
pressure  ten  miles  down  would  be  about  thirty-five 
tons  on  the  square  inch.  Even  a  slight  displacement 
of  one  extensive  surface  over  another,  the  sides  being 
pressed  together  with  a  force  of  thirty-five  tons  on  the 
square  inch,  would  be  an  operation  necessarily  accom- 
panied by  violence  greatly  exceeding  that  which  we 
might  expect  from  so  small  a  displacement  if  the  forces 
concerned  had  been  only  of  more  ordinary  magnitude. 
On  account  of  this  great  multiplication  of  the  intensity 
of  the  phenomenon,  merely  a  small  rearrangement  of 
the  rocks  in  the  crust  of  the  earth,  in  pursuance  of 
the  necessary  work  of  accommodating  its  volume  to 
the  perpetual  shrinkage,  might  produce  an  excessively 
violent  shock  extending  far  and  wide.  The  effect  of 
such  a  shock  would  be  propagated  in  the  form  01 
waves  through  the  globe,  just  as  a  violent  blow  given 
at  one  end  of  a  bar  of  iron  by  a  hammer  is  propagated 
through  the  bar  in  the  form  of  waves.  When  the 
effect  of  this  internal  adjustment  reaches  the  earth's 
surface,  it  will  sometimes  be  great  enough  to  be  per- 
ceptible in  the  shaking  it  gives  that  surface.  The 
shaking  may  be  so  violent  that  buildings  may  not  be 
able  to  withstand  it.  Such  is  the  phenomenon  of  an 
earthquake. 

Earthquakes  have  been  made  to  yield  testimony  of 
the  most  striking  character  with  regard  to  the  rigidity 
of  the  earth.  The  researches  we  are  now  to  describe 


WATCHING   EARTHQUAKES.  165 

are  mainly  due  to  Professor  Milne,  who,  having  enjoyed 
the  advantage  of  studying  earthquakes  in  their  natural 
home  in  Japan,  where  are  to  be  found  some  of  the 
most  earthquake-shaken  regions  of  this  earth,  has  now 
transferred  his  observations  of  these  phenomena  to  the 
more  peaceful  regions  of  the  Isle  of  Wight.  But  though 
the  Isle  of  Wight  is  perhaps  one  of  the  last  places  in 
the  world  to  which  anyone  who  desired  to  experience 
violent  earthquake  shocks  would  be  likely  to  go,  yet  by 
the  help  of  a  beautiful  apparatus  Professor  Milne  is 
actually  able  to  witness  important  earthquakes  that 
are  happening  all  over  the  world.  He  has  a  demon- 
stration of  these  earthquakes  in  the  indications  of  an 
extremely  sensitive  instrument  which  he  has  erected  in 
his  home  at  Shide. 

When  our  earth  is  shaken  by  one  of  those  occasional 
adjustments  of  the  crust  which  I  have  described,  the 
wave  that  spreads  like  a  pulsation  from  the  centre  of 
agitation  extends  all  over  our  globe  and,  indeed  I  may 
say,  is  transmitted  right  through  it.  At  the  surface 
lying  immediately  over  the  centre  of  disturbance  there 
will  be  a  violent  shock.  In  the  surrounding  country, 
and  often  over  great  distances,  the  earthquake  may 
also  be  powerful  enough  to  produce  destructive  effects. 
The  convulsion  may  also  be  manifested  over  a  far 
larger  area  of  country  in  a  way  which  makes  the 
shock  to  be  felt,  though  the  damage  wrought  may 
not  be  appreciable.  But  beyond  a  limited  distance 
from  the  centre  of  the  agitation  the  earthquake  will 
produce  no  destructive  effects  upon  buildings,  and  will 
not  even  cause  vibrations  that  would  be  appreciable 
to  ordinary  observation. 

This   earth   of  ours   may  transmit  from  an   earth- 
is 


166  THE    EARTH'S   BEGINNING. 

quake  pulses  of  a  very  distinct  and  definite  character, 
which  are  too  weak  to  be  perceived  by  our  unaided 
senses  ;  but,  just  as  the  microscope  will  render  objects 
visible  which  are  too  minute  to  be  perceived  without 
this  aid  to  the  ordinary  vision,  so  these  faint  earth-pulses 
may  be  rendered  perceptible  by  the  delicate  indica- 
tions of  an  instrument  which  perceives  and  records 
tremors  that  would  pass  unnoticed  by  our  ordinary 
observations.  The  ingenious  instrument  for  studying 
earthquakes  is  called  a  seismometer.  It  marks  on  a 
revolving  drum  of  paper  the  particulars  of  those 
infinitesimal  tremors  by  which  the  earth  is  almost 
daily  agitated  in  one  place  or  another. 

Let  us  suppose,  for  example,  that  an  earthquake 
occurs  in  Japan,  in  which  much  agitated  country  it 
is,  I  believe,  estimated  that  no  fewer  than  one  thou- 
sand earthquakes  of  varying  degrees  of  intensity  occur 
annually  in  one  district  or  another.  Let  us  suppose 
that  this  earthquake  behaves  as  serious  earthquakes 
usually  do;  that  it  knocks  down  buildings  and  monu- 
ments, causes  landslips,  raises  great  waves  in  the  sea 
and  hurls  them  as  inundations  on  the  land.  We  may 
also  suppose  that  it  causes  the  sad  loss  of  many  lives 
and  the  destruction  of  a  vast  quantity  of  property,  and 
that  its  energies  in  the  acutely  violent  form  extend 
over,  let  us  say,  an  area  of  a  hundred  square  miles. 
Beyond  that  area  of  greatest  destruction  such  an  earth- 
quake would  be  felt  over  a  great  extent  of  country  as 
a  shaking  more  or  less  vehement,  and  characteristic 
rumbling  sounds  would  be  heard.  But  the  intensity 
declines  with  the  distance,  and  we  may  feel  confident 
that  not  even  the  faintest  indications  of  the  earth- 
quake would  be  perceptible  by  the  unaided  senses  at 


THE   SEISMOMETER.  167 

a  thousand  miles  from  its  origin.  A  thousand  miles 
is,  however,  less  than  a  fifth  of  the  distance  between 
Tokio  and  Shide,  in  the  Isle  of  Wight,  measured  in 
a  great  circle  round  the  earth's  surface.  The  acutest 
sense  could  not  perceive  the  slightest  indication  of 
the  convulsion  in  Japan  at  even  half  the  distance  between 
these  two  places.  But  the  earth  transmits  so  faith- 
fully the  undulations  committed  to  its  care  that, 
though  the  intensity  may  have  declined  so  as  to  be 
no  longer  perceptible  to  sense,  it  is  still  possible  that 
they  may  be  shown,  and  shown  distinctly,  on  the  seis- 
mometer in  Professor  Milne's  laboratory,  even  after  a 
journey  of  five  thousand  miles.  This  instrument  not 
only  announces  that  an  earthquake  has  been  in  pro- 
gress some  little  time  previously,  but  the  recording 
pencil  reproduces  with  marvellous  fidelity  some  actual 
details  of  the  vibration.  The  movements  of  the  line 
up  and  down  on  the  revolving  drum  of  paper  show 
how  the  convulsions  succeed  each  other,  and  their 
varying  intensity.  Thus  Professor  Milne  is  enabled 
to  set  down  some  features  of  the  earthquake  long 
before  the  post  brings  an  account  of  the  convulsion 
from  the  unhappy  locality. 

Professor  Milne's  account  of  work  in  studying  earth- 
quakes has  the  charm  of  a  romance,  even  while  it 
faithfully  sets  out  the  facts  of  Nature.  I  have  sup- 
posed the  earthquake  to  take  place  in  Japan ;  but 
we  must  observe  that  the  seismometer  at  Shide  will 
also  take  account  of  considerable  earthquakes  in 
whatever  part  of  the  world  the  disturbance  may 
arise.  There  are,  for  example,  localities  in  the  West 
Indies  in  which  earthquakes  are  by  no  means  infre- 
quent, though  they  may  not  be  phenomena  of  almost 


168  THE   EARTH'S   BEGINNING. 

daily  occurrence,  as  they  are  in  Japan.  Every  con- 
siderable earthquake,  no  matter  where  its  centre 
may  lie,  produces  in  our  whole  globe  a  vibration 
or  a  tingle  which  is  sufficient  to  be  manifested 
by  the  delicate  indications  of  the  seismometer  at 
Shide.  Thus  this  instrument,  which  in  the  morning 
may  record  an  earthquake  from  Japan,  will  in  the 
afternoon  of  the  same  day  delineate  with  equal  fidelity 
an  earthquake  from  the  opposite  hemisphere  in  the 
neighbourhood  of  the  Caribbean  Sea. 

In  each  locality  in  which  earthquakes  are  chronic 
it  would  seem  as  if  there  must  be  some  particularly 
weak  spot  in  the  earth  some  miles  below  the  surface. 
A  shrinkage  of  the  earth,  in  the  course  of  the  in- 
cessant adjustment  between  the  interior  and  the 
exterior,  will  take  place  by  occasional  little  jumps  at 
this  particular  centre.  The  fact  that  there  is  this 
weak  spot  at  which  small  adjustments  are  possible 
may  provide,  as  it  were,  a  safety-valve  for  other 
places  in  the  same  part  of  the  world.  Instead  of  a 
general  shrinking,  the  materials  would  be  sufficiently 
elastic  and  flexible  to  allow  the  shrinking  for  a  very 
large  area  to  be  done  at  this  particular  locality.  In 
this  way  we  may  explain  the  fact  that  immense  tracts 
on  the  earth  are  practically  free  from  earthquakes  of 
a  serious  character,  while  in  the  less  fortunate  regions 
the  earthquakes  are  more  or  less  perennial. 

The  characteristics  of  an  earthquake  record,  a  seis- 
mogram,  if  we  give  it  the  correct  designation,  depend 
on  the  distance  of  the  origin  from  the  locality  where  the 
record  is  made.  The  length  of  the  journey,  as  might 
be  expected,  tells  on  the  character  of  the  inscription 
which  the  earthquake  waves  make  by  the  instrument. 


EARTHQUAKE    GROUPS.  169 

If,  for  instance,  the  first  intimation  of  a  large  earthquake 
received  at  Shide  precedes  the  second  by  about  thirty- 
five  minutes,  it  may  be  concluded  that  the  earthquake 
has  come  from  Japan. 

In  like  manner  the  shocks,  with  their  origin  in  the 
West  Indies,  will  proceed  from  their  particular  earth- 
quake centre,  and  consequently  all  the  earthquakes 
from  this  source  will  possess  a  characteristic  resem- 
blance. The  Japan  group  of  earthquakes  will  have, 
so  to  speak,  a  family  resemblance ;  and  the  Trinidad 
group  of  earthquakes,  though  quite  different  from  the 
Japan  group,  will  also  possess  a  family  resemblance. 
These  features  are  faithfully  transmitted  by  undula- 
tions through  the  earth  and  round  the  earth;  thus 
in  due  course  they  reach  the  Isle  of  Wight,  and  they 
are  reproduced  by  the  pencil  of  the  seismometer.  The 
different  earthquakes  of  a  family  may  differ  in  size, 
in  intensity,  and  undulation,  but  they  will  have  the 
features  appropriate  to  the  particular  group  from 
which  they  come.  From  long  experience  Professor 
Milne  has  become  so  familiar  with  the  lineaments  of 
these  earthquake  families,  that  in  his  study  at  Shide, 
as  he  looks  at  the  indications  of  his  instrument,  he 
is  able  to  say,  for  example,  "  Here  is  an  earthquake, 
and  it  is  a  little  earthquake  from  Japan;"  then  a 
little  later,  when  a  new  earthquake  begins,  he  will 
say,  "And  here  is  a  big  earthquake  from  Trinidad." 

Professor  Milne's  apparatus  has  brought  us  remark- 
able information  with  regard  to  the  interior  of  the 
earth.  The  story  which  we  have  to  tell  is  really  one 
of  the  most  astonishing  in  physical  science.  Let  us 
suppose  that  an  earthquake  originates  in  Japan.  We 
shall  assume  that  the  earthquake  is  a  vigorous  one, 


170  THE   EARTHS   BEGINNING. 

capable  of  producing  bold  and  definite  indications 
on  the  seismometer  even  in  the  Isle  of  Wight.  It  is  to 
be  noted  that  this  instrument  is  not  content  merely 
with  a  single  version  of  the  story  of  that  earthquake ; 
it  will  indeed  repeat  that  story  twice.  First  of 
all,  about  a  quarter  of  an  hour  after  a  shock  has 
taken  place  in  Japan,  the  pencil  of  the  seismometer 
commences  to  record.  But  this  record,  though  quite 
distinct,  is  not  so  boldly  indicated  as  the  subsequent 
records  of  the  same  event  which  will  presently  be 
received.  It  is  to  be  regarded  as  a  precursor.  After 
the  first  record  is  completed  there  is  a  pause  of  per- 
haps three-quarters  of  an  hour,  and  then  the  pencil 
of  the  seismometer  commences  again.  It  commences 
to  give  an  earthquake  record,  but  it  is  obviously  only 
a  second  version  of  the  same  earthquake.  For  the 
ups  and  downs  traced  by  the  pencil  are  just  the  same 
relatively  as  before.  The  picture  given  of  the  earth- 
quake is,  however,  on  a  much  larger  scale  than  the 
one  that  is  first  sent.  The  extent  of  the  shaking  of 
the  instrument  in  this  second  record  is  greater  than 
in  the  first,  and  all  the  details  are  more  boldly 
drawn. 

After  the  second  diagram  has  been  received,  there 
is  yet  another  pause,  which  may  be  perhaps  for  half 
an  hour.  Then,  by  the  same  pencil,  a  third  and  last 
version  is  conveyed  to  the  seismometer.  This  diagram 
is  not  quite  so  strong  as  the  last,  though  stronger 
than  the  first ;  in  it  again,  however,  the  faithful 
pencil  tells,  with  many  a  detail,  what  happened  in 
this  earthquake  at  Japan. 

We  have  first  to  explain  how  it  occurs  that  there 
are  three  versions  of  the  event,  for  it  need  hardly 


THE  THREE  RECORDS  EXPLAINED 


171 


be  said  that  the  same  earthquake  did  not  take 
place  three  different  times  over.  The  point  is  indeed 
a  beautiful  one.  The  explanation  is  so  astonishing 
that  we  should  hardly  credit  it  were  it  not  established 
upon  evidence  that  does  not  admit  of  a  moment's 
question. 

In  the  adjoining  diagram  we  represent  the  positiori 


Fig.  25. — EARTHQUAKE  ROUTES  FROM  JAPAN  TO  THE  ISLE  OF  WIGHT. 

of  Japan  at  one  side  of  the  earth,  and  the  Isle  of 
Wight  at  the  other.  When  the  earthquake  takes  place 
at  Japan  it  originates,  as  we  have  said,  a  series  of 
vibrations  through  our  globe.  We  must  here  dis- 
tinguish between  the  rocks — I  might  almost  say  the 
comparatively  pliant  rocks — which  form  the  earth's 
crust,  and  those  which  form  the  intensely  rigid  core 
of  the  interior  of  our  globe.  The  vibrations  which 
carry  the  tidings  of  the  earthquake  spread  through 


172  THE    EARTH'S    BEGINNING. 

the  rocks  on  the  surface,  from  the  centre  of  the 
disturbance,  in  gradually  enlarging  circles.  We  may 
liken  the  spread  of  these  vibrations  to  the  ripples  in 
a  pool  of  water  which  diverge  from  the  spot  where 
a  raindrop  has  fallen,  or  to  the  remarkable  air- 
waves from  Krakatoa,  to  which  we  shall  presently 
refer.  The  vibrations  transmitted  by  the  rocks  on 
the  surface,  or  on  the  floor  of  the  ocean,  will  carry 
the  message  all  over  the  earth.  As  these  rocks  are 
flexible,  at  all  events  by  comparison  with  the  earth's 
interior,  the  vibrations  will  be  correspondingly  large, 
and  will  travel  with  vigour  over  land  and  under 
sea.  In  due  time  they  reach  the  Isle  of  Wight, 
where  they  set  the  pencil  of  the  seismometer  at 
work.  But  there  are  different  ways  round  the  earth 
from  Japan  to  the  Isle  of  Wight.  There  is  the  most 
direct  route  across  Asia  and  Europe ;  there  is  also 
the  route  across  the  Pacific,  America,  and  the 
Atlantic.  The  vibrations  will  travel  by  both  routes, 
and  the  former  is  the  shorter  of  the  two.  The  vibra- 
tions which  take  the  first  route  through  the  crust 
of  the  earth's  surface  are  travelling  by  the  shorter 
distance ;  they  consequently  reach  Shide  first,  and 
render  their  version  of  what  has  happened.  But 
the  vibrations  which,  starting  from  the  centre  of  the 
disturbance,  move  through  the  earth's  crust  in  an 
opposite  direction  will  also  in  their  due  course  of 
expansion  reach  the  Isle  of  Wight.  They  will  have 
had  a  longer  journey,  and  will  consequently  be 
somewhat  enfeebled,  though  they  will  still  retain  tha 
characteristics  marking  the  particular  earthquake  centre 
from  which  they  arose. 

We   thus   account    for  both    the    second    and   the 


THE    SHORT   ROUTE.  173 

third  of  the  different  versions  of  the  earthquake  which 
are  received  at  Shide.  And  now  for  the  first  of  the 
three  versions.  This  is  the  one  which  is  of  special 
interest  to  us  at  present.  The  original  subterranean 
impulse  was,  as  we  have  seen,  propagated  through  the 
rocks  forming  the  earth's  crust.  Part  of  it,  however, 
entered  into  the  core  forming  the  earth's  interior. 
The  earthquake  had  the  power  not  only  of  shaking 
the  earth's  crust  all  over,  but  it  produced  the  astonish- 
ing effect  of  setting  the  whole  interior  of  our  globe  into 
a  tremble.  There  was  not  a  single  particle  of  our 
earth,  from  centre  to  surface,  which  was  not  made  to 
vibrate,  in  some  degree,  in  consequence  of  the  earth- 
quake. Certain  of  these  vibrations,  spreading  from 
the  centre  of  disturbance,  took  a  direct  course  to 
the  Isle  of  Wight,  right  through  the  globe.  They 
consequently  had  a  shorter  journey  in  travelling  from 
Tokio  to  Shide  than  those  which  went  round  the 
earth's  crust.  The  former  travelled  near  the  chord, 
while  the  latter  travelled  on  the  arc.  Even  for  this 
reason  alone  the  internal  vibrations  might  be  ex- 
pected to  accomplish  their  journey  more  rapidly 
than  the  superficial  movements.  With  the  same 
velocity  they  would  take  a  shorter  time  for  the 
journey.  There  is,  however,  another  reason  for  the 
lesser  time  taken  by  the  internal  vibrations.  Not 
only  is  the  journey  shorter,  but  the  speed  with  which 
these  vibrations  travel  through  the  solid  earth  is 
much  greater  than  the  speed  with  which  superficial 
vibrations  travel  through  the  crust.  It  has  been 
shown  that  the  average  velocity  of  these  vibrations 
when  travelling  through  the  centre  of  the  earth  is 
rather  more  than  ten  miles  a  second.  The  velocity 


174  THE   EARTH'S    BEGINNING. 

varies  with  the  square  root  of  the  depth,  and  near  the 
surface  it  is  not  two  miles  a  second. 

There  are  two  points  to  be  specially  noticed.  The 
vibrations,  which,  passing  through  the  earth's  interior 
with  a  high  velocity,  arrive  as  precursors,  make  a 
faithful  diagram,  but  only  on  a  very  small  scale. 
We  say  that  these  vibrations  have  but  small  ampli- 
tude This  shows  that  the  particles  in  the  earth's 
interior  are  not  much  displaced  by  the  earthquake,  as 
compared  with  those  on  the  earth's  crust,  and  this  is 
one  indication  of  the  effective  rigidity  of  the  earth. 
It  is  also  to  be  noted  that  the  great  speed  with 
which  the  vibrations  traverse  the  solid  earth  is  a  con- 
sequence of  the  extreme  rigidity  of  our  globe.  These 
vibrations  travel  more  rapidly  through  the  earth  than 
they  would  do  through  a  bar  of  solid  steel.  In  other 
words,  we  have  here  a  proof  that,  under  the  influence 
of  the  tremendous  pressures  characteristic  of  the  earth's 
interior,  the  material  of  which  that  earth  is  composed, 
notwithstanding  the  high  temperature  to  which  it  is 
raised,  possesses  a  rigidity  which  is  practically  greater 
than  that  of  steel  itself. 

This  is  perhaps  the  most  striking  testimony  that 
can  be  borne  to  the  rigidity  of  our  globe;  but  we 
must  not  imagine  that  we  are  dependent  solely  upon 
the  phenomena  of  earthquakes  for  the  demonstration 
of  this  important  point ;  there  are  other  proofs.  It 
can  be  shown  that  the  ebb  and  flow  of  the  tides  on 
our  coasts  would  be  very  different  from  that  which 
they  actually  are  were  it  not  that  the  earth  behaves  as 
a  rigid  globe.  It  has  also  been  demonstrated  that 
certain  astronomical  phenomena  connected  with  the 
way  in  which  the  earth  turns  round  on  its  axis 


SHOWING       LOCALITIES     OF      EARTHQUAKES 


THE    SOLID    EARTH.  175 

would  not  be  the  same  as  we  actually  find  them  to 
be  if  the  earth  were  not  solid  in  its  interior. 

The  result  of  these  investigations  is  to  show  that, 
though  this  globe  of  ours  must  be  excessively  hot 
inside,  so  hot  indeed  that  at  ordinary  pressures  even 
the  most  refractory  solids  would  be  liquefied  or 
vaporised,  yet  under  the  influence  of  the  pressure  to 
which  its  materials  are  subjected  the  behaviour  of 
that  globe  is  as  that  of  the  most  rigidly  solid  body. 

Happily  in  this  country  we  do  not  often  experience 
earthquakes  other  than  delicate  movements  shown  by 
the  record  of  the  seismometer.  But  though  most  of 
us  live  our  lives  without  ever  having  felt  an  earthquake 
shock,  yet  earthquakes  do  sometimes  make  themselves 
felt  in  Great  Britain.  The  map  we  here  give,  which 
was  drawn  by  Professor  J.  P.  O'Reilly,  indicates  the 
localities  in  England  in  which  from  time  to  time  earth- 
quake shocks  have  been  experienced. 

The  internal  heat  of  the  earth,  derived  from  the 
primaeval  nebula,  is  in  no  way  more  strikingly  illus- 
trated than  by  the  phenomena  of  volcanoes.  We  have 
shown  in  this  chapter  that  there  is  no  longer  any 
reason  to  believe  that  the  earth  is  fluid  in  its  interior. 
The  evidence  has  proved  that,  under  the  extraordinary 
pressure  which  prevails  in  the  earth,  the  materials  in 
the  central  portions  of  our  globe  behave  with  the 
characteristics  of  solids  rather  than  of  liquids.  But 
though  this  applies  to  the  deep-seated  regions  of  our 
globe,  it  need  not  universally  apply  at  the  surface  or 
within  a  moderate  depth  from  the  surface.  When  the 
circumstances  are  such  that  the  pressure  is  relaxed, 
then  the  heat  is  permitted  to  exercise  its  property  of 
transforming  the  solids  into  liquids.  Masses  of  matter 


176  TEE   EARTH'S   BEGINNING. 

near  the  earth's  crust  are  thus,  in  certain  circum- 
stances, and  in  certain  localities,  transformed  into  the 
fluid  or  viscid  form.  In  that  state  they  may  issue  from 
a  volcano  and  flow  in  sluggish  currents  as  lava. 

There  has  been  much  difference  of  opinion  as  to 
the  immediate  cause  of  volcanic  action,  but  there  can 
be  little  doubt  that  the  energy  which  is  manifested 
in  a  volcanic  eruption  has  been  originally  derived  in 
some  way  from  the  contraction  of  the  primaeval  nebula. 
The  extraordinary  vehemence  that  a  volcanic  eruption 
sometimes  attains  may  be  specially  illustrated  by  the 
case  of  the  great  eruption  of  Krakatoa.  It  is,  indeed, 
believed  that  in  the  annals  of  our  earth  there  has 
been  no  record  of  a  volcanic  eruption  so  vast  as  that 
which  bears  the  name  of  this  little  island  in  far  Eastern 
seas,  ten  thousand  miles  from  our  shores. 

Until  the  year  1883  few  had  ever  heard  of  Krakatoa. 
It  was  unknown  to  fame,  as  are  hundreds  of  other 
gems  of  glorious  vegetation  set  in  tropical  waters.  It 
was  not  inhabited,  but  the  natives  from  the  surround- 
ing shores  of  Sumatra  and  Java  used  occasionally  to 
draw  their  canoes  up  on  its  beach,  while  they  roamed 
through  the  jungle  in  search  of  the  wild  fruits  that 
there  abounded.  Geographers  in  early  days  hardly 
condescended  to  notice  Krakatoa;  the  name  of  the 
island  on  their  maps  would  have  been  far  longer  than 
the  island  itself.  It  was  known  to  the  mariner  who 
navigated  the  Straits  of  Sunda,  for  it  was  marked  on  his 
charts  as  one  of  the  perils  of  the  intricate  navigation 
in  those  waters.  It  was  no  doubt  recorded  that  the 
locality  had  been  once,  or  more  than  once,  the  seat  of 
an  active  volcano.  In  fact,  the  island  seemed  to  owe 
its  existence  to  some  frightful  eruption  of  bygone- 


KRAKATOA.  177 

days;  but  for  a  couple  of  centuries  there  had  been  no 
fresh  outbreak.  It  almost  seemed  as  if  Krakatoa  might 
be  regarded  as  a  volcano  that  had  become  extinct.  In 
this  respect  it  would  only  be  like  many  other  similar 
objects  all  over  the  globe,  or  like  the  countless  extinct 
volcanoes  all  over  the  moon. 

In  1883  Krakatoa  suddenly  sprang  into  notoriety. 
Insignificant  though  it  had  hitherto  seemed,  the  little 
island  was  soon  to  compel  by  its  tones  of  thunder  the 
whole  world  to  pay  it  instant  attention.  It  was  to 
become  the  scene  of  a  volcanic  outbreak  so  appalling 
that  it  is  destined  to  be  remembered  throughout  the 
ages.  In  the  spring  of  that  year  there  were  symptoms 
that  the  volcanic  powers  in  Krakatoa  were  once  more 
about  to  awake  from  the  slumber  that  had  endured  for 
many  generations.  Notable  warnings  were  given.  Earth- 
quakes were  felt,  and  deep  rumblings  proceeded  from  the 
earth,  showing  that  some  disturbance  was  in  preparation, 
and  that  the  old  volcano  was  again  to  burst  forth  after 
its  long  period  of  rest.  At  first  the  eruption  did  not 
threaten  to  be  of  any  serious  type ;  in  fact,  the  good 
people  of  Batavia,  so  far  from  being  terrified  at  what 
was  in  progress  in  Krakatoa,  thought  the  display  was 
such  an  attraction  that  they  chartered  a  steamer  and 
went  forth  for  a  pleasant  picnic  to  the  island.  Many 
of  us,  I  am  sure,  would  have  been  delighted  to  have 
been  able  to  join  the  party  who  were  to  witness  so 
interesting  a  spectacle.  With  cautious  steps  the  more 
venturesome  of  the  excursion  party  clambered  up  the 
sides  of  the  volcano,  guided  by  the  sounds  which  were 
issuing  from  its  summit.  There  they  beheld  a  vast 
column  of  steam  pouring  forth  with  terrific  noise  from 
a  profound  opening  about  thirty  yards  in  width. 


178  THE    EARTH'S    BEGINNING. 

As  the  summer  of  this  dread  year  advanced  the 
vigour  of  Krakatoa  steadily  increased,  the  noises  became 
more  and  more  vehement ;  these  were  presently  audible 
on  shores  ten  miles  distant,  and  then  twenty  miles 
distant;  and  still  those  noises  waxed  louder  and  louder, 
until  the  great  thunders  of  the  volcano,  now  so  rapidly 
developing,  astonished  the  inhabitants  that  dwelt  over 
an  area  at  least  as  large  as  Great  Britain.  And  there 
were  other  symptoms  of  the  approaching  catastrophe. 
With  each  successive  convulsion  a  quantity  of  fine 
dust  was  projected  aloft  into  the  clouds.  The  wind 
could  not  carry  this  dust  away  as  rapidly  as  it  was 
hurled  upwards  by  Krakatoa,  and  accordingly  the 
atmosphere  became  heavily  charged  with  suspended 
particles.  A  pall  of  darkness  thus  hung  over  the 
adjoining  seas  and  islands.  Such  was  the  thickness 
and  the  density  of  these  atmospheric  volumes  of 
Krakatoa  dust  that,  for  a  hundred  miles  around,  the 
darkness  of  midnight  prevailed  at  midday.  Then 
the  awful  tragedy  of  Krakatoa  took  place.  Many 
thousands  of  the  unfortunate  inhabitants  of  the 
adjacent  shores  of  Sumatra  and  Java  were  destined 
never  to  behold  the  sun  again.  They  were  presently 
swept  away  to  destruction  in  an  invasion  of  the  shore 
by  the  tremendous  waves  with  which  the  seas  sur- 
rounding Krakatoa  were  agitated. 

Gradually  the  development  of  the  volcanic  energy 
proceeded,  and  gradually  the  terror  of  the  inhabitants 
of  the  surrounding  coasts  rose  to  a  climax.  July  had 
ended  before  the  manifestations  of  Krakatoa  had  at- 
tained their  full  violence.  As  the  days  of  August  passed 
by  the  spasms  of  Krakatoa  waxed  more  and  more 
vehement.  By  the  middle  of  that  month  the  panic 


180  THE    EAETH'S    BEGINNING. 

was   widespread,  for   the  supreme   catastrophe  was  at 
hand. 

On  the  night  of  Sunday,  August  26th,  1883,  the  black- 
ness of  the  dust-clouds,  now  much  thicker  than  ever  in 
the  Straits  of  Sunda  and  adjacent  parts  of  Sumatra  and 
Java,  was  only  occasionally  illumined  by  lurid  flashes 
from  the  volcano.  The  Krakatoan  thunders  were  on 
the  point  of  attaining  their  complete  development.  At 
the  town  of  Batavia,  a  hundred  miles  distant,  there  was 
no  quiet  that  night.  The  houses  trembled  with  the 
subterranean  violence,  and  the  windows  rattled  as  if 
heavy  artillery  were  being  discharged  in  the  streets. 
And  still  these  efforts  seemed  to  be  only  rehearsing  for 
the  supreme  display.  By  ten  o'clock  on  the  morning 
of  Monday,  August  27th,  1883,  the  rehearsals  were 
over  and  the  performance  began.  An  overture,  con- 
sisting of  two  or  three  introductory  explosions,  was 
succeeded  by  a  frightful  convulsion  which  tore  away  a 
large  part  of  the  island  of  Krakatoa  and  scattered  it  to 
the  winds  of  heaven.  In  that  final  effort  all  records 
of  previous  explosions  on  this  earth  were  completely 
broken. 

This  supreme  effort  it  was  which  produced  the 
mightiest  noise  that,  so  far  as  we  can  ascertain,  has 
ever  been  heard  on  this  globe.  It  must  have  been 
indeed  a  loud  noise  which  could  travel  from  Krakatoa 
to  Batavia  and  preserve  its  vehemence  over  so  great 
a  distance ;  but  we  should  form  a  very  inadequate 
conception  of  the  energy  of  the  eruption  of  Krakatoa 
if  we  thought  that  its  sounds  were  heard  by  those 
merely  a  hundred  miles  off.  This  would  be  little  indeed 
compared  with  what  is  recorded,  on  testimony  which 
it  is  impossible  to  doubt. 


THE     EARLY     STAGE     OF     THE     ERUPTION     OF     KRAKATOA. 

(From  a  Photograph  taken  on  May  27th,  1883.) 


THE 

;• 


A    MIGHTY   SOUND.  181 

Westward  irom  Krakatoa  stretches  the  wide  expanse 
of  the  Indian  Ocean.  On  the  opposite  side  from  the 
Straits  of  Sunda  lies  the  island  of  Rodriguez,  the  dis- 
tance from  Krakatoa  being  almost  thrp.p. 


It  has  been  proved  by  evidence  which  cannot  be  doubted 
that  the  thunders  of  the  great  volcano  attracted  the 
attention  of  an  intelligent  coastguard  on  Rodriguez,  who 
carefully  noted  the  character  of  the  sounds  and  the  time 
of  their  occurrence.  He  had  heard  them  just  four  hours 
after  the  actual  explosion,  for  this  is  the  time  the  sound 
occupied  on  its  journey. 

We  shall  better  realise  the  extraordinary  vehemence 
of  this  tremendous  noise  if  we  imagine  a  similar  event 
to  take  place  in  localities  more  known  to  most  of  us 
than  are  the  far  Eastern  seas. 

If  Vesuvius  were  vigorous  enough  to  emit  a  roar  like 
Krakatoa,  how  great  would  be  the  consternation  of  the 
world  !  Such  a  report  might  be  heard  by  King  Edward 
at  Windsor,  and  by  the  Czar  of  all  the  Russias  at 
Moscow.  It  would  astonish  the  German  Emperor  and 
all  his  subjects.  It  would  penetrate  to  the  seclusion  of 
the  Sultan  at  Constantinople.  Nansen  would  still  have 
been  within  its  reach  when  he  was  furthest  north,  near 
the  Pole.  It  would  have  extended  to  the  sources  of 
the  Nile,  near  the  Equator.  It  would  have  been  heard 
by  Mohammedan  pilgrims  at  Mecca.  It  would  have 
reached  the  ears  of  exiles  in  Siberia.  No  inhabitant  of 
Persia  would  have  been  beyond  its  range,  while  pas- 
sengers on  half  the  liners  crossing  the  Atlantic  would 
also  catch  the  mighty  reverberation. 

Or,  to  take  another  illustration  that  I  gave  some 
years  ago  in  the  Young  People's  Journal,  let  us  suppose 
that  a  similar  earth-shaking  event  took  place  in  a  central 

13 


182  THE   EARTH'S    BEGINNING. 

position  in  the  United  States.  Let  us  say,  for  example, 
that  an  explosion  occurred  at  Pike's  Peak  as  resonant  as 
that  from  Krakatoa.  It  would  certainly  startle  not  a  little 
the  inhabitants  of  Colorado  far  and  wide.  The  ears  of 
dwellers  in  the  neighbouring  States  would  receive  a  con- 
siderable shock.  With  lessening  intensity  the  sound 
would  spread  much  further  around — indeed,  it  might  be 
heard  all  over  the  United  States.  The  sonorous  waves 
would  roll  over  to  the  Atlantic  coast,  they  would  be 
heard  on  the  shores  of  the  Pacific.  Florida  would  not 
be  too  far  to  the  south,  nor  Alaska  too  remote  to  the 
north.  If,  indeed,  we  could  believe  that  the  sound 
would  travel  as  freely  over  the  great  continent  as  it  did 
across  the  Indian  Ocean,  then  we  may  boldly  assert 
that  every  ear  in  North  America  might  listen  to  the 
thunder  from  Pike's  Peak,  if  it  rivalled  Krakatoa. 
The  reverberation  might  even  be  audible  by  skin- 
clad  Eskimos  amid  the  snows  of  Greenland,  and  by 
naked  Indians  sweltering  on  the  Orinoco.  Can  we 
doubt  that  Krakatoa  made  the  greatest  noise  that 
has  ever  been  recorded  ? 

Among  the  many  other  incidents  connected  with 
this  explosion,  I  may  specially  mention  the  wonderful 
system  of  divergent  ripples  that  started  in  our  atmo- 
sphere from  the  point  at  which  the  eruption  took 
place.  I  have  called  them  ripples,  from  the  obvious 
resemblance  which  they  bear  to  the  circular  expanding 
ripples  produced  by  raindrops  which  fall  upon  the  still 
surface  of  water.  But  it  would  be  more  correct  to 
say  that  these  objects  were  a  series  of  great  undulations 
which  started  from  Krakatoa  and  spread  forth  in  ever- 
enlarging  circles  through  our  atmosphere.  The  initial 
impetus  was  so  tremendous  that  these  waves  spread  for 


184  THE   EARTH'S    BEGINNING. 

hundreds  and  thousands  of  miles.  They  diverged,  in 
fact,  until  they  put  a  mighty  girdle  round  the  earth, 
on  a  great  circle  of  which  Krakatoa  was  the  pole.  The 
atmospheric  waves,  with  the  whole  earth  now  well  in 
their  grasp,  advanced  into  the  opposite  hemisphere.  In 
their  further  progress  they  had  necessarily  to  form 
gradually  contracting  circles,  until  at  last  they  con- 
verged to  a  point  in  Central  America,  at  the  very 
opposite  point  of  the  diameter  of  our  earth,  eight 
thousand  miles  from  Krakatoa.  Thus  the  waves  com- 
pletely embraced  the  earth.  Every  part  of  our  atmo- 
sphere had  been  set  into  a  tingle  by  the  great  eruption. 
In  Great  Britain  the  waves  passed  over  our  heads, 
the  air  in  our  streets,  the  air  in  our  houses,  trembled 
from  the  volcanic  impulse.  The  very  oxygen  supplying 
our  lungs  was  responding  also  to  the  supreme  con- 
vulsion which  took  place  ten  thousand  miles  away. 
It  is  needless  to  object  that  this  could  not  have 
taken  place  because  we  did  not  feel  it.  Self-registering 
barometers  have  enabled  these  waves  to  be  followed 
unmistakably  all  over  the  globe. 

Such  was  the  energy  with  which  these  vibrations 
were  initiated  at  Krakatoa,  that  even  when  the  waves 
thus  arising  had  converged  to  the  point  diametrically 
opposite  in  South  America  their  vigour  was  not  yet 
exhausted.  The  waves  were  then,  strange  to  say, 
reflected  back  from  their  point  of  convergence  to 
retrace  their  steps  to  Krakatoa.  Starting  from  Central 
America,  they  again  described  a  series  of  enlarging 
circles,  until  they  embraced  the  whole  earth.  Then, 
advancing  into  the  opposite  hemisphere,  they  gradually 
contracted  until  they  had  regained  the  Straits  of 
Sunda,  from  which  they  had  set  forth  about  thirty-six 


TEN  MILES    OVERHEAD.  185 

hours  previously.  Here  was,  indeed,  a  unique  experience. 
The  air- waves  had  twice  gone  from  end  to  end  of  this 
globe  of  ours.  Even  then  the  atmosphere  did  not  sub- 
side until,  after  some  more  oscillations  of  gradually 
fading  intensity,  at  last  they  became  evanescent. 

But,  besides  these  phenomenal  undulations,  this 
mighty  incident  at  Krakatoa  has  taught  us  other 
lessons  on  the  constitution  of  our  atmosphere.  We 
previously  knew  little,  or  I  might  almost  say  nothing, 
as  to  the  conditions  prevailing  above  the  height  of  ten 
miles  overhead.  We  were  almost  altogether  ignorant 
of  what  the  wind  might  be  at  an  altitude  of,  let  us 
say,  twenty  miles.  It  was  Krakatoa  which  first  gjave 
us  a  little  information  which  was  greatly  wanted.  How 
could  we  learn  what  winds  were  blowing  at  a  height 
four  times  as  great  as  the  loftiest  mountain  on  the 
earth,  and  twice  as  great  as  the  loftiest  altitude  to 
which  a  balloon  has  ever  soared  ?  We  'could  neither 
see  these  winds  nor  feel  them.  How,  then,  could  we 
learn  whether  they  really  existed  ?  No  doubt  a  straw 
will  show  the  way  the  wind  blows,  but  there  are  no 
straws  up  there.  There  was  nothing  to  render  the 
winds  perceptible  until  Krakatoa  came  to  our  aid. 
Krakatoa  drove  into  those  winds  prodigious  quantities 
of  dust.  Hundreds  of  cubic  miles  of  air  were  thus 
deprived  of  that  invisibility  which  they  had  hitherto 
maintained.  They  were  thus  compelled  to  disclose 
those  movements  about  which,  neither  before  nor  since, 
have  we  had  any  opportunity  of  learning. 

With  eyes  full  of  astonishment  men  watched  those 
vast  volumes  of  Krakatoa  dust  start  on  a  tremendous 
journey.  Westward  the  dust  of  Krakatoa  took  its 
way.  Of  course,  everyone  knows  the  so-called  trade- 


186  THE    EARTH'S   BEGINNING. 

winds  on  our  earth's  surface,  which  blow  steadily  in 
fixed  directions,  and  which  are  of  such  service  to  the 
mariner.  But  there  is  yet  another  constant  wind.  We 
cannot  call  it  a  trade-wind,  for  it  never  has  rendered, 
and  never  will  render,  any  service  to  navigation.  It 
was  first  disclosed  by  Krakatoa.  Before  the  occur- 
rence of  that  eruption  no  one  had  the  slightest  sus- 
picion that  far  up  aloft,  twenty  miles  over  our  heads, 
a  mighty  tempest  is  incessantly  hurrying  with  a 
speed  much  greater  than  that  of  the  awful  hurricane 
which  once  laid  so  large  a  part  of  Calcutta  on  the 
ground,  and  slew  so  many  of  its  inhabitants.  Fortu- 
nately for  humanity,  this  new  trade-wind  does  not 
come  within  less  than  twenty  miles  of  the  earth's 
surface.  We  are  thus  preserved  from  the  fearful 
destruction  that  its  unintermittent  blasts  would  pro- 
duce, blasts  against  which  no  tree  could  stand,  and 
which  would,  in  ten  minutes,  do  as  much  damage  to 
a  city  as  would  the  most  violent  earthquake.  When 
this  great  wind  had  become  charged  with  the  dust 
of  Krakatoa,  then,  for  the  first  and,  I  may  add,  for 
the  only  time,  it  stood  revealed  to  human  vision. 
Then  it  was  seen  that  this  wind  circled  round  the 
earth  in  the  vicinity  of  the  Equator,  and  completed  its 
circuit  in  about  thirteen  days. 

Please  observe  the  contrast  between  this  wind  of 
which  we  are"  now  speaking  and  the  waves  to  which 
we  have  just  referred.  The  waves  were  merely  un- 
dulations or  vibrations  produced  by  the  blow  which 
our  atmosphere  received  from  the  explosion  of  Kra- 
katoa, and  these  waves  were  propagated  through  the 
atmosphere  much  in  the  same  way  as  sound  waves 
are  propagated.  Indeed,  these  waves  moved  with  the 


STRANGE    SIGHTS  IN    THE   SKY.  187 

same  velocity  as  sound.  But  the  current  of  air  of 
which  we  are  now  speaking  was  not  produced  by 
Krakatoa ;  it  existed  from  all  time,  before  Krakatoa 
was  ever  heard  of,  and  it  exists  at  the  present 
moment,  and  will  doubtless  exist  as  long  as  the 
earth's  meteorological  arrangements  remain  as  they 
are  at  present.  All  that  Krakatoa  did  was  simply  to 
provide  the  charges  of  dust  by  which  for  one  brief 
period  this  wind  was  made  visible. 

In  the  autumn  of  1883  the  newspapers  were  full 
of  accounts  of  strange  appearances  in  the  heavens. 
The  letters  containing  these  accounts  poured  in  upon 
us  from  residents  in  Ceylon;  they  came  from  resi- 
dents in  the  West  Indies,  and  from  other  tropical 
places.  All  had  the  same  tale  to  tell.  Sometimes 
experienced  observers  assured  us  that  the  sun  looked 
blue ;  sometimes  we  were  told  of  the  amazement  with 
which  people  beheld  the  moon  draped  in  vivid  green. 
Other  accounts  told  of  curious  halos,  and,  in  short,  of 
the  signs  in  the  sun,  the  moon,  and  the  stars,  which 
were  exceedingly  unusual,  even  if  we  do  not  say  that 
they  were  absolutely  unprecedented. 

Those  who  wrote  to  tell  of  the  strange  hues  that 
the  sun  manifested  to  travellers  in  Ceylon,  or  to 
planters  in  Jamaica,  never  dreamt  of  attributing  the 
phenomena  to  Krakatoa,  many  thousands  of  miles 
away.  In  fact,  these  observers  knew  nothing  at  the 
time  of  the  Krakatoa  eruption,  and  probably  few  of 
them,  if  any,  had  ever  heard  that  such  a  place  existed. 
It  was  only  gradually  that  the  belief  grew  that  these' 
phenomena  were  due  to  Krakatoa.  But  when  the 
accounts  were  carefully  compared,  and  when  the  dates 
were  studied  at  which  the  phenomena  were  witnessed  in 


188  THE    EARTH'S    BEGINNING. 

the  various  localities,  it  was  demonstrated  that  these 
phenomena,  notwithstanding  their  worldwide  distribu- 
tion, had  certainly  arisen  from  the  eruption  in  this 
little  island  in  the  Straits  of  Sunda.  It  was  most 
assuredly  Krakatoa  that  painted  the  sun.  and  the 
moon,  and  produced  the  other  strange  and  weird  phe- 
nomena in  the  tropics. 

After  a  little  time  we  learned  what  had  actually 
happened.  The  'dust  manufactured  by  the  supreme 
convulsion  was  whirled  round  the  earth  in  the  mighty 
atmospheric  current  into  which  the  volcano  dis- 
charged it.  As  the  dust-cloud  was  swept  along  by 
this  incomparable  hurricane,  it  showed  its  presence  in 
the  most  glorious  manner  by  decking  the  sun  and 
the  moon  in  hues  of  unaccustomed  splendour  and 
beauty.  The  blue  colour  in  the  sky  under  ordinary 
circumstances  is  due  to  particles  in  the  air,  and 
when  the  ordinary  motes  of  the  sunbeam  were  rein- 
forced by  the  introduction  of  the  myriads  of  motes 
produced  by  Krakatoa,  even  the  sun  itself  sometimes 
showed  a  blue  tint.  Thus  the  progress  of  the  great 
dust-cloud  was  traced  out  by  the  extraordinary  sky 
effects  it  produced,  and  from  the  progress  of  the 
dust-cloud  we  inferred  the  movements  of  the  in- 
visible air  current  which  carried  it  along.  Nor  need 
it  be  thought  that  the  quantity  of  material  projected 
from  Krakatoa  should  have  been  inadequate  to  pro- 
duce effects  of  this  worldwide  description.  Imagine 
that  the  material  which  was  blown  to  the  winds 
of  heaven  by  the  supreme  convulsion  of  Krakatoa  - 
could  be  all  recovered  and  swept  into  one  vast 
heap.  Imagine  that  the  heap  were  to  have  its  bulk 
measured  by  a  vessel  consisting  of  a  cube  one  mile 


THE   BEAUTIFUL    SUNSETS.  189 

long,  one  mile  broad,  and  one  mile  deep;  it  has 
been  estimated  that  even  this  prodigious  vessel  would 
have  to  be  filled  to  the  brim  at  least  ten  times  before 
all  the  products  of  Krakatoa  had  been  measured. 

It  was  in  the  late  autumn  of  1883  that  the 
marvellous  series  of  celestial  phenomena  connected 
with  the  great  eruption  began  to  be  displayed  in 
Great  Britain.  Then  it  was  that  the  glory  of  the 
ordinary  sunsets  was  enhanced  by  a  splendour  which 
has  dwelt  in  the  memory  of  all  those  who  were  per- 
mitted to  see  them.  The  frontispiece  of  this  volume 
contains  a  view  oi  the  sunset  as  seen  at  Chelsea 
at  4.40  p.m.  on  November  26th,  1883.  The  picture 
was  painted  from  nature  by  Mr.  W.  Ascroft,  and 
is  given  in  the  great  work  on  Krakatoa  which  was 
published  by  the  Royal  Society.  There  is  not  the 
least  doubt  that  it  was  the  dust  from  Krakatoa 
which  produced  the  beauty  of  those  sunsets,  and 
so  long  as  that  dust  remained  suspended  in  our 
atmosphere,  so  long  were  strange  signs  to  be  wit- 
nessed in  the  heavenly  bodies.  But  the  dust  which 
had  been  borne  with  unparalleled  violence  from  the 
interior  of  the  volcano,  the  dust  which  had  been 
sho.t  aloft  by  the  vehemence  of  the  eruption  to  an 
altitude  of  twenty  miles,  the  dust  which  had  thus 
been  whirled  round  and  round  our  earth  for  perhaps 
a  dozen  times  or  more  in  this  air  current,  which 
carried  it  round  in  less  than  a  fortnight,  was  en- 
dowed with  no  power  to  resist  for  ever  the  law  of 
gravitation  which  bids  it  fall  to  the  earth.  It  there- 
fore gradually  sank  downwards.  Owing,  however,  to 
the  great  height  to  which  it  had  been  driven,  owing 
to  the  impetuous  nature  of  the  current  by  which 


190  THE   EARTH'S    BEGINNING. 

it  was  hurried  along,  and  owing  to  the  exceed- 
ingly minute  particles  of  which  it  was  composed,  the 
act  of  sinking  was  greatly  protracted.  Not  until 
two  years  after  the  original  explosion  had  all  the 
particles  with  which  the  air  was  charged  by  the 
great  eruption  finally  subsided  on  the  earth. 

At  first  there  were  some  who  refused  to  be- 
lieve that  the  glory  of  the  sunsets  in  London  could 
possibly  be  due  to  a  volcano  in  the  Straits  of  Sunda, 
at  a  distance  from  England  which  was  but  little 
short  of  that  of  Australia.  But  the  gorgeous  pheno- 
mena in  England  were  found  to  be  simultaneous  with 
similar  phenomena  in  other  places  all  round  the 
earth.  Once  again  the  comparison  of  dates  and  other 
circumstances  proved  that  Krakatoa  was  the  cause  of 
these  exceptional  and  most  interesting  phenomena. 
Tennyson,  ever  true  to  nature,  records  the  event  in 
immortal  verse — 

"  Had  the  fierce  ashes  of  some  fiery  peak 
Been  hurled  so  high  they  ranged  around  the  world, 
For  day  by  day  through  many  a  blood-red  eve 
The  wrathful  sunset  glared." 


CHAPTER    X. 

SPIRAL   AND   PLANETARY    NEBULA. 

A  Substitute  for  History— Photograph  of  the  Great  Spiral  taken  at  the 
Lick  Observatory — Solar  System  Relations  Unimportant — Chaotic 
Nebulae — Lord  Rosse's  Great  Discovery — Dr.  Roberts'  Photographs 
— The  Astonishing  Discovery  of  Professor  Keeler — The  Perspective 
of  the  Spirals — The  Spiral  Nebulae  are  not  Gaseous— The  Spiral 
is  a  Nebula  in  an  advanced  Stage  of  Development — Character  of 
the  Great  Nebula  in  Andromeda. 

IN  a  great  college  in  America  a  new  educational  ex- 
periment has  been  tried  with  some  success.  Instead 
of  the  instruction  in  history  which  students  receive 
in  most  other  institutions,  an  attempt  has  been  made 
in  this  college  to  give  instruction  in  a  very  different 
manner,  which  it  is  believed  will  not  be  of  less  edu- 
cational value  than  the  more  ordinary  processes  of 
teaching.  In  the  course  of  study  to  which  I  am 
now  referring  the  student  is  invited  to  consider,  not 
so  much  the  history  of  the  development  of  the  Consti- 
tution of  one  particular  country,  as  to  make  a  broad 
survey  of  the  different  Constitutions  under  which  the 
several  countries  of  the  world  are  at  this  moment 
governed.  The  promoters  of  this  scheme  believe 
that  many  of  the  intellectual  advantages  which  are 


192  THE   EARTH'S   BEGINNING. 

ordinarily  expected  to  be  gained  by  the  study  of  the 
history  of  one  country  may  be  secured  equally  well 
by  studying  only  existing  conditions,  provided  that 
attention  is  given  to  several  countries  which  have 
arrived  at  different  stages  of  civilisation. 

Without  attempting  to  say  how  far  the  study  of 
the  existing  Constitutions  of  France  and  Germany, 
America  and  Australia,  Turkey  and  India,  Morocco 
and  Fiji,  might  be  justly  used  to  supersede  the  study 
of  English  history,  it  may  at  least  be  urged  that  if 
we  had  no  annals  from  which  history  could  be 
compiled  it  might  be  instructive  to  employ  such  a 
substitute  for  historical  studies  as  is  here  suggested. 
This  is,  indeed,  the  course  which  we  are  compelled  to 
take  in  our  study  of  that  great  chapter  in  earth- 
history  which  we  are  discussing  in  these  pages.  It 
is  obvious  from  the  nature  of  the  case  that  it  can 
never  be  possible  for  us  to  obtain  direct  testimony  as 
to  what  occurred  in  the  bringing  together  of  the 
materials  of  this  globe.  We  must,  therefore,  look 
abroad  through  the  universe,  and  see  whether  we  can 
find,  from  the  study  of  other  systems  at  present  in 
various  stages  of  their  evolution,  illustrations  of  the 
incidents  which  we  may  presume  to  have  occurred  in 
the  early  stages  of  our  own  history. 

If  Kant  had  never  lived,  if  Laplace  had  never  an- 
nounced his  Nebular  Theory,  if  the  discoveries  of  Sir 
William  Herschel  had  not  been  made,  I  still  venture 
to  think  that  a  due  consideration  of  the  remarkable 
photograph  of  the  famous  Great  Spiral,  which  was 
obtained  at  the  famous  Lick  Observatory  in  California, 
would  have  suggested  the  high  probability  of  that 
doctrine  which  we  describe  as  the  Nebular  Theory. 


Fig-.  28. — THE  GREAT  SPIRAL  NEBULA  (Lick  Observatory ). 

(From  the  Royal  Astronomical  Series.) 


194  THE   EARTH'S    BEGINNING. 

If  an  artist  thoroughly  versed  in  the  great  facts  of 
astronomy  had  been  commissioned  to  represent  the 
nebular  origin  of  our  system  as  perfectly  as  a  highly 
cultivated  yet  disciplined  imagination  would  permit, 
I  do  not  think  he  could  have  designed  anything 
which  could  answer  the  purpose  more  perfectly  than 
does  that  picture  which  is  now  before  us.  We  might 
wish  indeed  that  Kant  and  Laplace  and  Herschel 
could  have  lived  to  see  this  marvellous  natural 
illustration  of  their  views,  for  photographs  were  of 
course  unthought  of  in  those  days,  and,  I  need  hardly 
say,  that  for  any  one  celestial  nebula  that  could  have 
been  known  in  the  times  of  Laplace,  hundreds  are  now 
within  the  reach  of  astronomers. 

We  entreat  special  attention  to  this  picture  which 
Nature  has  herself  given  us,  and  which  represents 
what  we  may  not  unreasonably  conclude  to  be  a 
system  in  a  state  of  formation.  Let  me  say  at  once 
that  our  solar  system,  however  imposing  it  may  be 
from  our  point  of  view,  is  but  of  infinitesimal  import- 
ance as  compared  with  the  system  which  is  here  in  the 
course  of  development.  It  is  sometimes  urged  that  it 
is  difficult  to  imagine  how  a  system  so  large  as  ours 
could  have  been  produced  by  condensation  from  a 
primaeval  nebula.  The  best  answer  is  found  in  the 
lact  that  the  Great  Spiral  now  before  us  may  be 
considered  to  exhibit  at  this  very  moment  a  system 
in  actual  evolution,  the  central  body  of  which  is 
certainly  thousands  of  times,  and  not  improbably 
millions  of  times,  greater  than  the  sun,  and  of  which 
the  attending  planets  or  other  revolving  bodies,  are 
framed  on  a  scale  immensely  transcending  that  of 
even  Jupiter  himself.  The  details  of  this  remarkable 


THE    GREAT   SPIRAL.  195 

nebula  seem  to  illustrate  those  particular  features 
which  had  been  previously  assigned  to  the  primaeval 
nebula  of  our  system,  long  before  any  photograph  was 
available  for  the  purpose  of  their  study. 

In  the  Great  Nebula  in  Orion,  to  which  we  have 
already  referred,  as  well  as  in  many  other  similar 
objects  which  we  might  also  have  adduced,  the 
nebulous  material  from  which  after  long  ages  new 
systems  may  be  the  result,  was  shown  in  an  extremely 
chaotic  state.  It  was  little  more  than  an  irregular 
stain  of  light  on  the  sky.  But  in  the  picture 
of  the  Great  Spiral  which  is  before  us  (Fig.  28) 
it  is  manifest  that  the  evolution  of  the  system  has 
reached  an  advanced  stage ;  such  considerable  pro- 
gress has  been  made  in  the  actual  formation  that  the 
final  form  seems  to  be  shadowed  forth.  The  lumi- 
nosity is  no  longer  diffused  in  a  chaotic  condition  ; 
it  has  formed  into  spirals,  and  become  much  con- 
densed at  the  centre  and  somewhat  condensed  in  other 
regions.  As  we  now  see  it,  the  object  seems  to  re- 
present a  system  much  more  advanced  in  its  forma- 
tion than  any  of  the  other  great  nebulae  with  which 
we  have  compared  it.  In  comparison  with  it  the 
evolution  of  such  an  object  as  the  Great  Nebula  in 
Orion  can  hardly  be  said  to  have  begun.  But  in 
the  Great  Spiral  many  portions  of  the  nebula  have 
already  become  outlined  into  masses  which,  though 
still  far  from  resembling  the  planets  in  the  solar  system, 
have  at  least  made  some  approach  thereto  while  the 
central  portions  are  being  drawn  together,  just  as  we 
may  conceive  the  great  primaeval  fire-mist  to  have 
drawn  together  in  the  actual  formation  of  the  sun. 

The   famous  nebula  which   we  are  discussing,  and 


196 


THE    EARTH'S    BEGINNING. 


The     Great     Bear. 


The  Great  Spiral,  NebuUa. 


oCor  Caroii 


Fig.  29. — How  TO  FIND  THE  GREAT  SPIRAL  NEBULA. 

which  is  generally  known  as  the  Great  Spiral,  is  found 
in  the  constellation  of  Canes  Venatici,  very  near  the 
end  star  in  the  tail  of  the  Great  Bear,  and  one-fourth 
of  the  way  from  it  to  Cor  Caroii.  It  will  be  easy 
to  find  it  from  the  indication  given  in  the  adjoining 
Fig.  29.  As  a  nebulous  spot  it  is  an  object  which  can  be 
seen  with  any  moderately  good  telescope,  but  to  detect 
those  details  which  indicate  the  spiral  structure  de- 
mands an  instrument  of  first-class  power.  This  object  had 
indeed  been  studied  by  many  astronomers  before  Lord 
Kosse  turned  his  colossal  reflector  upon  it.  Then  it  was 
that  the  wonderful  whirlpool  structure  was  first  discovered, 
and  thus  the  earliest  spiral  nebula  became  known. 

In   those  days  there  were  few  telescopes   of  great 
power,   and  none   of  those  instruments  appeared  able 


LORD   ROSSES   DISCOVERT.  197 

to  deal  with  this  nebula  sufficiently  to  reveal  its  spiral 
character.  The  announcement  of  the  discovery  of  the 
spiral  constitution  of  this  object  was  therefore  received 
with  incredulity  by  some  astronomers,  who  believed, 
or  professed  to  believe,  that  the  spiral  lines  of  nebulous 
matter  which  Lord  Rosse  described  so  faithfully,  existed 
only  in  the  imagination  of  the  astronomer.  Indeed, 
in  one  notable  instance,  it  was  alleged  that  these  features 
were  to  be  attributed  to  actual  imperfections  in  the 
unrivalled  telescope.  The  incredulity  widely  prevalent 
in  the  middle  of  the  last  century  about  the  existence  of 
the  spiral  nebulae  may  be  paralleled  by  the  incredulity 
about  other  discoveries  in  more  recent  years.  When  a 
highly  skilled  observer,  using  an  instrument  of  adequate 
power,  and,  it  may  be,  enjoying  unequalled  opportunities 
for  good  work,  testifies  to  certain  discoveries ;  when 
he  has  employed  in  the  verification  of  his  observations 
the  skill  and  experience  that  years  of  practice  have 
procured  for  him,  it  is  futile  for  those  who  have  not 
the  like  opportunities,  either  from  the  want  of  instru- 
ments of  adequate  power  or  from  climatic  difficulties,  to 
deny  the  truth  of  discoveries  because  they  are  not  able 
to  verify  them.  It  was  absurd  for  astronomers  to  refuse 
assent  to  the  great  discoveries  of  Lord  Rosse  simply 
because  instruments  inferior  to  his  would  not  show 
the  spiral  structure. 

In  due  time,  one  astronomer  after  another  began  to 
admit  that  possibly  the  remarkable  form  which  Lord 
Rosse  announced  as  characteristic  of  some  nebulae  might 
not  be  a  mere  figment  of  the  imagination.  The  complete 
vindication  of  Lord  Rosse's  great  discovery  was  not> 
however,  attained  until  that  wonderful  advance  in  the 
arts  of  astronomy  when  the  photographic  plate  was 
14 


198  THE   EARTH'S    BEGINNING. 

called  in  to  supplement,  or  rather  vastly  to  extend,  the 
powers  of  the  eye.  Dr.  Isaac  Roberts  not  only  showed 
by  a  magnificent  photograph  that  the  Great  Spiral 
discovered  by  Lord  Rosse  was  just  as  Lord  Rosse  had 
described  it,  he  not  only  showed  that  the  other  spirals 
announced  by  Lord  Rosse  were  equally  entitled  to  the 
name,  but,  with  the  newly  acquired  powers  that  the 
photographic  plate  placed  at  his  disposal,  he  was  able  to 
show  that  many  other  nebulae,  which  had  been  frequently 
observed  and  had  even  been  sketched,  possessed  further 
features  too  faint  and  delicate  to  be  seen  by  any  human 
eye,  even  with  the  help  of  the  most  powerful  telescope. 
These  further  features  were  discovered  because  they 
came  within  the  ken  of  the  intensely  acute  perception  of 
the  photographic  plate.  On  the  plate  these  features 
which  the  camera  showed,  were  added  to  those  which 
the  eye  had  already  perceived,  and  when  these  additions 
were  made  it  was  not  infrequently  found  that  the 
nebula  assumed  the  form  of  a  spiral.  But  the  most 
remarkable  circumstance  has  still  to  be  added.  Some 
of  the  plates  exposed  by  Dr.  Roberts  show  clear  and 
unmistakable  photographs  of  spiral  nebulae  as  exquisite 
in  detail  as  the  Great  Spiral  itself,  but  yet  so  faint  that 
they  have  never  been  seen  by  the  eye  in  any  telescope 
whatever,  though  they  could  not  elude  the  photographic 
plate.  Thus,  Dr.  Roberts  not  only  confirmed  in  the 
most  splendid  manner  that  really  great  discovery  of  the 
spiral  nebulas  of  which  the  honour  belongs  to  Lord  Rosse, 
but  the  eminent  photographic  astronomer  added  many 
other  spirals  of  the  greatest  interest  to  the  list  of  those 
objects  which  Lord  Rosse  had  himself  given. 

Though  these  discoveries  placed  the  fact  of  the  exist- 
ence of  spiral  nebulae  in  an  impregnable  position,  and 


PROFESSOR    KEELER.  199 


Fig.  30. — A  GROUP  or  NEBULAE  (Lord  tiosse). 
(3440,  3445  in  n.g.c.) 

(From  the  Scientific  Transactions  of  the  Royal  Dublin  Society.) 

though  they  greatly  increased  the  interest  with  which 
astronomers  study  such  objects,  yet  another  step  had  to 
be  taken  before  the  spiral  nebula  attained  the  position 
of  extraordinary  importance  as  a  celestial  object  which 
must  now  be  acknowledged  to  be  its  due. 

We  have  already  had  occasion  (page  67)  to  mention 
the  marvellous  discoveries  of  nebulse  which  the  lamented 
Professor  Keeler  made  with  the  Crossley  Reflector  at 
the  Lick  Observatory.  We  have  explained  that  his  dis- 
coveries have  shown  the  number  of  nebula3  in  the  heavens 
to  be  probably  at  least  twenty  times  that  which  previous 
observations  would  have  authorised  us  in  asserting. 
The  mere  announcement  that  120,000  new  nebulae 
were  within  the  reach  ot  a  photographic  plate  attached 
to  the  Crossley  Reflector,  would,  by  itself,  have  been  a 


200  THE   E  ART  IT  8   BEGINNING. 

statement  so  remarkable  as  to  command  the  immediate 
attention  of  the  scientific  world.  But  the  interest  of  even 
this  statement  shrinks  to  unimportance  relatively  to 
the  further  fact  which  Professor  Keeler  has  added.  I 
do  not  know,  in  the  annals  of  astronomy,  a  pronounce- 
ment of  greater  interest,  certainly  none  of  more  import- 
ance for  our  present  purpose,  than  the  statement 
that  of  the  120,000  new  nebulae,  at  least  half  are 
spirals.  Here  is  indeed  a  stupendous  revolution  in 
our  knowledge  of  the  celestial  objects.  Fifty  years 
ago  Lord  Rosse  announced  the  discovery  of  a  spiral 
nebula,  and  the  existence  of  this  spiral  was  doubted 
at  first,  though  it  was  gradually  conceded  at  last.  Now 
we  have  the  announcement,  on  the  unchallenged  evi- 
dence of  the  photographic  plate  itself,  that  to  all 
appearances  there  are  at  least  60,000  spiral  nebulae  in 
the  heavens.  It  is,  alas !  too  true  that  Professor  Keeler 
did  not  live  long  enough  to  enumerate  all  those 
nebulae  himself,  and,  indeed,  they  have  not  so  far 
been  actually  counted,  but  to  those  who  will  study 
Professor  Keeler's  papers,  the  evidence  of  the  sub- 
stantial accuracy  of  the  statement  is  incontestable. 

And  astonishing  as  this  statement  may  be,  we  have 
still  to  add  that,  in  face  of  the  actual  facts,  it  may 
be  regarded  as  even  a  moderate  estimate  of  the 
abundance  of  spirals  in  the  universe.  We  must  re- 
member that  a  spiral  nebula  is  a  flat  object  with  long 
arms  extending  from  it  which  lie  nearly  in  the  same 
plane.  If  we  are  actually  to  see  that  such  an  object 
is  spiral,  it  is  necessary  for  it  to  be  turned  squarely 
towards  the  earth.  If  the  object  be  too  much  fore- 
shortened, it  is  quite  plain  that  we  can  hardly  expect 
to  detect  its  spiral  character.  It  is  also  obvious, 


RAT   NEBULA.  201 


Fig.  31. — A  RAY  NEBULA  (Lord  Rome). 

(3628  in  n.g.c.) 
(From  the  Scientific  Transactions  of  the  Royal  Dublin  Society.) 

if  the  spiral  happens  to  be  turned  edgeways  towards 
us,  that  then  its  spiral  form  cannot  be  seen ;  it  would 
merely  appear  as  what  astronomers  often  call  a  ray. 
In  the  enumeration  of  the  spirals  it  is  therefore 
only  possible  for  us  to  include  those  which  happen 
to  be  so  far  squarely  turned  towards  the  earth  as 
to  make  their  spiral  character  unmistakeable.  We 
might,  therefore,  reasonably  expect  that  the  numbers 
of  spiral  nebulae  actually  counted  would  fall  short 
of  the  reality.  We  know  that  there  are  many  nebulae 
of  a  somewhat  elliptical  shape  (Fig.  31).  There  are  also 
many  nebulae  that  look  like  long  rays  (Fig.  30).  Those 
who  are  familiar  with  the  appearance  of  nebulae  in 
great  telescopes  will  recall  at  once  the  numerous 
spindle-shaped  objects  ol  this  class.  It  can  hardly 


202  THE    EARTHS    BEGINNING. 

be  doubted  that  many  of  the  nebulae,  more  or  less 
oval  in  form,  and  also  these  rays  or  the  spindle-shaped 
objects  so  frequently  seen  in  good  telescopes  (Fig.  33) 
are  in  reality  spiral  nebulse,  which  are  turned  not 
squarely  towards  us,  but  which  we  are  merely  looking 
at  more  or  less  edgewise,  so  that  they  have  been  fore- 
shortened enough  to  hide  their  peculiar  structure  (Figs. 
34,  35).  Taking  these  considerations  into  account,  it 
becomes  obvious  that  the  estimate  of  Professor  Keeler 
as  to  the  number  of  spiral  nebulse  in  the  heavens,  vast 
as  that  estimate  seems,  may  still  fall  short  of  the  truth. 
Thus  we  are  led  to  one  of  the  most  remarkable  con- 
clusions of  modern  astronomy,  that  the  spiral  nebula, 
next  to  a  star  itself,  may  be  the  most  characteristic 
object  in  the  sidereal  heavens. 

In  treating  of  the  nebulse  in  Chapter  IY.  we  ex- 
plained those  fundamental  features  of  the  different 
spectra  which  make  it  possible  to  discriminate  with 
confidence  between  a  nebula  which  is  purely  gaseous 
and  a  nebula  which  cannot  be  so  described.  As  the 
spiral  nebulse  form  a  class  characterised  among  all 
the  other  nebulse  by  the  possession  of  a  very  par- 
ticular structure,  it  is  interesting  to  enquire  what 
evidence  the  spectrum  gives  with  regard  to  the  nature 
of  the  material  which  enters  into  the  constitution  of 
the  nebulse'  which  belong  to  this  strongly-marked  group. 
I  do  not  mean  to  say  that  all  the  60,000  spirals  have 
been  examined  with  the  spectroscope,  but,  as  already 
explained  on  page  67,  a  sufficient  number  have  been 
examined  to  decide  the  question.  We  learn  from  Pro- 
fessor Scheiner,  a  well-known  authority  on  astronomical 
spectroscopy,  that  the  spectra  of  spirals  are  generally 
found  to  be  continuous ;  in  other  words,  we  learn  that 


STAGES    OF  NEBULJE.  203 

a  spiral  nebula  is  not  gaseous.  It  does  not  consist, 
like,  for  example,  the  nebula  in  Orion,  of  vaporous 
matter  in  a  state  of  incandescence. 

A  nebula  or  a  nebulous-looking  object  which  does 
not  give  a  spectrum  of  bright  lines,  but  which  does 
give  a  continuous  spectrum,  is  not  infrequently  set 
down  as  being  merely  a  cluster  of  stars.  This  is 
undoubtedly  a  true  statement  with  regard  to  some  of 
these  nebulous  objects,  but  it  is  not  true  with  regard 
to  all.  It  is  much  more  reasonable  to  suppose  that 
the  greater  part  of  the  materials  of  the  spiral  nebulae, 
though  certainly  not  in  the  form  of  gas,  are  still  not 
condensed  into  objects  large  enough  to  entitle  them  to 
be  called  stars.  It  must  be  remembered  that  when 
an  object  of  a  gaseous  nature  has  lost  heat  by  radia- 
tion, and  has  begun  to  draw  itself  together,  the  gas 
condenses  into  particles  which  constitute  small  portions 
of  liquid  or  solid,  just  as  the  vapour  of  water  in 
the  atmosphere  condenses  into  the  beads  of  water 
that  form  the  clouds  in  our  own  sky.  These  small 
objects,  even  if  incandescent,  would  no  longer  radiate 
light  with  the  characteristics  of  a  gaseous  nebula.  The 
light  they  would  emit  would  be  of  the  same  character 
as  that  dispensed  from  the  particles  of  carbon  in  the 
solar  photosphere  to  which  the  sun  owes  its  light. 
Radiation  from  such  a  source  would  give  light  with  a 
continuous  spectrum,  like  that  from  the  sun  or  a  star. 

From  the  fact  that  the  spectra  of  the  spiral  nebulae 
are  continuous,  we  may  infer  that,  though  these 
nebulae  have  reached  an  advanced  stage  in  their 
development,  they  have  not  always,  and,  perhaps, 
not  generally,  attained  to  the  stage  in  which  con- 
densation transformed  them  into  a  cluster  of  actual 


204  THE    EARTH'S    BEGINNING. 

stars.  They  have,  however,  reached  a  stage  in  their 
progress  towards  those  systems  of  large  bodies  that 
they  are  ultimately  to  become.  The  character  of  its 
spectrum  may  show  us  that  the  spiral  nebula  is  not 
very  young,  that  it  has  attained  a  considerable  age 
in  its  evolution  as  compared  with  other  nebulse  which 
do  not  show  the  spiral  character  and  which  hav,e  a 
gaseous  spectrum.  The  importance  of  this  considera- 
tion will  be  made  apparent  in  the  next  chapter,  when 
we  discuss  the  dynamical  conditions  to  which  a  spiral 
nebula  must  submit. 

But  there  is  no  reason  to  doubt  that  some  of 
the  spiral  nebulae  may  be  in  reality  star-clusters,  in 
which  there  are  aggregations  of  myriads  of  points,, 
each  justly  entitled  by  its  dimensions  and  its  lustre 
to  be  regarded  as  a  real  star.  The  great  nebula  in 
Andromeda  seems  to  be  a  greatly  foreshortened  spiral. 
This,  at  least,  is  the  interpretation  which  may  perhaps 
be  most  reasonably  given  to  Dr.  Roberts'  famous 
photograph  of  this  splendid  object.  The  spectrum  of 
the  Andromeda  nebula  has  been  photographed  by 
Scheiner  after  a  protracted  exposure  of  seven  and  a 
half  hours.  That  spectrum  showed  no  trace  of  bright 
lines,  thus  proving  that  there  is  no  discernible  in- 
candescent gas  in  the  nebula  of  Andromeda.  It 
gives  practically  a  continuous  spectrum,  across  which 
some  broad  bands  can  be  recognised.  It  was  in- 
teresting to  compare  this  spectrum  of  the  great 
nebula  in  Andromeda  with  the  solar  spectrum  seen 
by  the  same  apparatus  and  under  the  same  conditions. 
Professor  Scheiner  announces  that  there  was  a  re- 
markable coincidence  between  the  two,  and  he  draws 
the  inference  that  the  stars  which  enter  into  the 


THE    THEORY    CONFIRMED. 


205 


Fig.  32.— PORTION  OF  THE  MILKY  WAY  (NEAR  MESSIER  II.). 

(Photographed  by  Projessor  E.  E.  Barnard.) 
(From   the    Royal  Astronomical  Society  Series.) 

nebula    in    Andromeda    are   stars    of    that   particular 
type  to  which  the  sun  belongs. 

But  we  have  now  to  point  out  how  the  recent  study 
of  nebulae  has  afforded  a  yet  more  striking  confirmation 


206  THE    EARTH'S    BEGINNING. 

of  the  nebular  theory.  Laplace  showed  how  a  gradually 
condensing  nebula  might  have  formed  a  sun  and  a 
system  of  planets.  It  might,  however,  have  been  urged 
as  an  objection  in  his  time,  that  this  suggestion  for  the 
origin  of  the  solar  system  was  a  purely  speculative  idea, 
and  that  Nature  did  not  permit  us  to  behold,  at  present, 
any  evolutions  in  progress  which  might  illustrate  the 
actual  process  of  the  evolution  of  the  solar  system.  But 
this  objection  can  be  no  longer  urged,  now  that  the 
spiral  nebulae  are  known.  Had  Laplace  known  of  the 
spiral  nebulae  he  would,  I  doubt  not,  have  found  in 
them  the  most  striking  illustration  of  the  operation  of 
evolution  on  a  gigantic  scale.  They  would  have  pro- 
vided him  with  admirable  arguments  in  support  of  the 
nebular  theory.  It  is  possible  that  they  might  also  have 
provided  suggestions  as  to  the  details  of  the  evolution, 
which  he  had  not  anticipated.  But  Laplace  did  not 
know  of  such  objects,  and  we  can  only  deplore  the 
loss  of  the  instructive  lessons  which  his  incomparable 
genius  would  have  derived  from  them. 

We  must,  however,  admit  that  the  lessons  as  to  the 
origin  of  the  solar  system,  derived  from  the  spiral  nebulae, 
must  be  received  with  due  limitation.  We  may  say  at 
once  that  the  great  spiral  nebulae  do  not  appear  to  be 
evolving  into  systems  like  the  sun  and  planets ;  their 
work  is  of  a  higher  order  of  magnitude  altogether.  The 
great  spiral  nebulas  seem  to  be  more  analogous  to  galaxies, 
like  the  Milky  Way  (Fig.  32),  than  to  solar  systems. 
The  spiral  nebula  instead  of  being  described  as  a  system, 
should  perhaps  be  described  as  a  system  of  systems.  If 
the  solar  system  were  drawn  to  scale  on  the  photograph 
of  the  Great  Spiral  (Fig.  28)  the  orbit  of  Neptune  would 
not  be  larger  than  the  smallest  recognisable  dot. 


CHAPTER    XL 

THE    UNERRING    GUIDE. 

The  Solar  System — Orbits  nearly  Plane— Satellites,  Saturn's  Ring, 
Spiral  Nebulae— An  Explanation  of  this  Tendency  of  a  System 
towards  Flatness— The  Energy  of  a  System— Loss  of  Energy  by 
Collision  and  Tidal  Action — A  System  within  a  System— Movements 
of  Translation  and  Movements  of  Rotation — The  General  Law  of 
Conservation  of  Moment  of  Momentum  —  Illustrations  of  the 
Principle — The  Conception  of  the  Principal  Plane — The  Utility  of 
this  principle  arises  from  its  independence  of  Collisions  or  Friction 
— Nature  does  not  do  Things  inBnitely  Improbable — The  Decline  of 
Energy  and  the  Preservation  of  Moment  of  Momentum — Explanation 
of  the  Motions  in  one  Plane  and  in  the  same  Direction— The 
Satellites  of  Uranus— The  Rotation  of  Uranus— Why  the  Orbits  are 
not  exactly  in  the  same  Plane — The  Evolution  of  a  Nebula — The 
Inevitable  Tendency  towards  the  Spiral — The  Explanation  of  the 
Spiral. 

WE  have  to  consider  in  this  chapter  the  light  which  the 
laws  of  mathematics  throw  upon  certain  features  which 
are  possessed  by  a  very  large  number  of  celestial  objects. 
Let  us  first  describe,  as  clearly  as  the  circumstances  will 
permit,  the  nature  of  these  common  features  to  which 
we  now  refer,  and  of  which  mathematics  will  suggest 
the  explanation. 

We  shall  begin  with  our  solar  system,  in  which  the 
earth  describes  an  orbit  around  the  sun.  That  orbit 
is  contained  within  a  plane,  which  plane  passes  through 


208  THE    EARTH'S    BEGINNING. 

the  centre  of  the  sun.  We  may  neglect  for  the 
present  the  earth's  occasional  slight  deviations  from 
this  plane  which  are  caused  by  the  attractions  of 
the  other  planets.  If  we  consider  the  other  bodies  of 
our  system,  such,  for  instance,  as  Venus  or  Jupiter,  we 
find  that  the  orbit  of  Venus  also  lies  in  a  plane,  and 
that  plane  also  passes  through  the  centre  of  the  sun. 
The  orbit  of  Jupiter  is  found  to  be  contained  within  a 
plane,  and  it,  too,  passes  through  the  sun's  centre.  Each 
of  the  remaining  planets  in  like  manner  is  found  to 
revolve  in  an  orbit  which  is  contained  in  a  plane,  and  all 
these  planes  have  one  common  point,  that  point  being 
the  centre  of  the  sun. 

It  is  a  remarkable  fact  that  the  mutual  inclinations 
are  very  small,  so  that  the  several  planes  are  nearly 
coincident.  If  we  take  the  plane  of  our  earth's  orbit, 
which  we  call  the  ecliptic,  as  the  standard,  then  the 
greatest  inclination  of  the  orbit  of  any  other  important 
planet  is  seven  degrees,  which  is  found  in  the  case 
of  Mercury.  The  inclinations  to  the  ecliptic  of  the 
planes  of  the  orbits  of  a  few  of  the  asteroids  are  much 
more  considerable ;  to  take  an  extreme  case,  the  orbit  of 
Pallas  is  inclined  at  an  angle  of  no  less  than  thirty-four 
degrees.  It  must,  however,  be  remembered  that  the 
asteroids  are  very  small  objects,  as  the  collective  masses 
of  the  five  hundred  which  are  at  present  known  would 
amount  to  no  more  than  an  unimportant  fraction  of  the 
mass  of  one  of  the  great  planets  of  our  system.  Three- 
fourths  of  the  asteroids  have  inclinations  under  ten 
degrees.  We  may,  therefore,  leave  these  bodies  out 
of  consideration  for  the  present,  though  we  may  find 
occasion  to  refer  to  them  again  later  on.  Still  less  need 
we  pay  attention  at  present  to  the  comets,  for  though 


PLANETS    AND    THEIR    SATELLITES.         209 

these  bodies  belong  to  our  system,  and  though  they 
move  in  plane  orbits,  which  like  the  orbits  of  the  planets 
pass  through  the  centre  of  the  sun,  yet  their  orbits  are 
inclined  at  angles  of  very  varying  magnitudes.  Indeed, 
we  cannot  detect  any  tendency  in  the  orbits  of  comets 
to  approximate  to  the  plane  of  the  ecliptic.  The 
masses  of  comets. are,  however,  inconsiderable  in  com- 
parison with  the  robust  globes  which  form  the  planets, 
while  the  origin  of  comets  has  been  apparently  so 
different  from  that  of  the  planets,  that  we  may  leave 
them  out  of  consideration  in  our  present  argument. 
There  is  nothing  in  the  motion  of  either  asteroids  or 
comets  to  invalidate  the  general  proposition  which 
affirms,  that  the  planes  of  the  orbits  of  the  heaviest 
and  most  important  bodies  in  the  solar  system  are 
very  nearly  coincident. 

Many  of  the  planets  are  accompanied  by  satellites, 
and  these  satellites  revolve  round  the  planets,  just  as 
the  planet  accompanied  by  its  satellites  revolves  round 
the  sun.  The  orbit  of  each  satellite  is  contained  within 
a  plane,  and  that  plane  passes  through  the  centre  of  the 
planet  to  which  it  is  appended.  We  thus  have  a  system 
of  planes  appropriate  to  the  satellites,  just  as  there  is  a 
system  of  planes  appropriate  to  the  planets.  The  orbits 
of  the  satellites  of  each  planet  are  very  nearly  in  the 
same  plane,  with  notable  exceptions  in  the  cases  of 
Uranus  and  Neptune,  which  it  will  be  necessary  to  con- 
sider at  full  length  later  on.  This  plane  is  very  nearly 
coincident  with  the  planes  in  which  the  planets  them- 
selves move.  Omitting  the  exceptions,  which  are  unim- 
portant as  to  magnitude,  though  otherwise  extremely 
interesting  and  instructive,  the  fundamental  character- 
istic of  the  movements  of  the  principal  bodies  in  our 


210  THE    EARTH'S    BEGINNING. 

system  is  that  their  orbits  are  nearly  parallel  to  the 
same  plane.  We  draw  an  average  plane  through  these 
closely  adjacent  planes  and  we  term  it  the  principal 
plane  of  our  system.  It  is  not,  indeed,  coincident  with 
the  plane  of  the  orbit  of  any  one  planet,  yet  the  actual 
plane  of  the  orbit  of  every  important  planet,  and  of 
the  important  satellites,  lies  exceedingly  close  to  this 
principal  plane.  This  is  a  noteworthy  circumstance  in 
the  arrangement  of  the  planetary  system,  and  we  expect 
that  it  must  admit  of  some  physical  explanation. 

When  we  look  into  the  details  of  the  planetary 
groups  composing  the  solar  system,  we  find  striking 
indications  of  the  tendency  of  the  orbits  of  the  bodies  in 
each  subordinate  system  to  become  adjusted  to  a  plane. 
The  most  striking  instance  is  that  exhibited  by"  the  Rings 
of  Saturn.  It  has  been  demonstrated  that  these  wonder- 
ful rings  are  composed  of  myriads  of  separate  particles. 
Each  of  these  particles  follows  an  independent  orbit 
round  Saturn.  Each  such  orbit  is  contained  in  a  plane, 
and  all  these  planes  appear,  so  far  as  our  observations 
go,  to  be  absolutely  coincident.  It  is  further  to  be  noted 
that  the  plane,  thus  remarkably  related  to  the  system  of 
rings  revolving  around  Saturn,  is  substantially  identical 
with  the  plane  in  which  the  satellites  of  Saturn  them- 
selves revolve,  and  this  plane  again  is  inclined  at  an 
angle  no  greater  than  twenty-eight  degrees  to  the  plane 
of  the  ecliptic,  and  close  to  that  in  which  Saturn  itself 
revolves  around  the  sun. 

Overlooking,  as  we  may  for  the  present,  the  varieties 
in  detail  which  such  natural  phenomena  present,  we 
may  say  that  the  most  noticeable  characteristic  of  the 
revolutions  in  the  solar  system  is  expressed  by  the  state- 
ment that  they  lie  approximately  in  the  same  plane. 


THE    SPIRAL    NEBULA    FLAT. 


211 


Fig.    33. — A  SPIRAL  NEBULA  SEEN  EDGEWISE  (n.g.c.  3628;  in  Leo). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

We  shall  also  find  that  this  tendency  of  the  move- 
ments in  a  system  to  range  themselves  in  orbits  which 
lie  in  the  same  plane,  is  exhibited  in  other  parts  of  the 
universe.  Let  us  consider  from  this  point  of  view  the 
spiral  nebulae,  those  remarkable  objects  which,  in  the 
last  chapter,  we  have  seen  to  be  so  numerous  and  so 
characteristic.  It  is  obvious  that  a  spiral  nebula  must 
be  a  flat  object.  Its  thickness  is  small  in  comparison 
with  its  diameter.  When  a  spiral  nebula  is  looked  at  edge- 
wise (Fig.  45),  then  it  seems  long  and  thin,  so  much  so 
that  it  presents  the  appearance  of  a  ray  such  as  we  have 
shown  in  Fig.  33,  which  represents  a  type  of  object 


212 


THE    EARTH'S    BEGINNING. 


Fig.  34. — A  FORE-SHOKTENED  SPIRAL  (n.g.c.  3198;  in  Ursa  Major). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

very  familiar  to  those  astronomers  who  are  acquainted 
with  nebulae.  The  observations  of  these  objects  seem 
consistent  only  with  the  supposition  that  there  is  a 
tendency  in  the  materials  which  enter  into  a  spiral 
nebula  to  adapt  their  movements  to  a  particular  plane, 
just  as  there  is  a  tendency  for  the  objects  in  Saturn's 
ring  to  remain  in  a  particular  plane,  and  just  as  there  has 
been  a  tendency  among  the  bodies  belonging  to  the  solar 
system  themselves  to  revolve  in  a  particular  plane.  And, 
remembering  that  there  seems  excellent  reason  to  believe 
that  the  spiral  nebulae  exhibiting  this  characteristic 


THE    TENDENCY   OF  A    SYSTEM.  213 


Fig.  35.— EDGE  VIEW  OF  A  SPIRAL  BOLDLY  SHOWN  (n.g.c.  4565; 

in  Coma  Berenices). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

are  to  be  reckoned  in  scores  of  thousands,  it  is  evident 
that  the  fundamental  feature  in  which  they  all  agree 
must  be  one  of  very  great  importance  in  the  universe. 
We  may  mention  yet  one  more  illustration  of  the 
remarkable  tendency,  so  frequently  exhibited  by  an 
organised  system  in  space,  to  place  its  parts  ultimately 
in  or  near  the  same  plane,  or  at  all  events,  to  assume  a 
shape  of  which  one  dimension  is  small  in  comparison 
with  the  two  others.  We  have,  in  the  last  chapter, 
referred  to  the  Milky  Way,  and  we  have  alluded  to  the 

15 


214  THE    EARTHS    BEGINNING. 

significance  of  the  obvious  fact  that,  however  the  mass 
of  stars  which  form  the  Milky  Way  may  be  arranged, 
they  are  so  disposed  that  the  thickness  of  the  mass  is- 
certainly  much  less  than  its  two  other  dimensions. 
Herschel's  famous  illustration  of  a  grindstone  to  repre- 
sent the  shape  of  the  Milky  Way  will  at  least  serve  to 
illustrate  the  form  which  we  are  now  considering. 

When  we  meet  with  a  characteristic  form  so  widely 
diffused  through  the  universe,  exhibited  not  only  in  the 
systems  attending  on  the  single  planets,  not  only  in  the 
systems  of  planets  which  revolve  round  a  single  sun, 
but  also  in  that  marvellous  aggregation  of  innumerable 
suns  which  we  find  in  the  Milky  Way,  and  in  scores 
of  thousands  of  nebulae  in  all  directions,  at  all  distances, 
and  apparently  of  every  grade  of  importance,  we  are 
tempted  to  ask  whether  there  may  not  be  some  physical 
explanation  of  a  characteristic  so  universal  and  so 
remarkable. 

Let  us  see  whether  mathematics  can  provide  any 
suggestion  as  to  the  cause  of  this  tendency  towards 
flatness  which  seems  to  affect  those  systems  in  the 
universe  which  are  sufficiently  isolated  to  escape  from 
any  large  disturbance  of  their  parts  by  outside  inter- 
ference. We  must  begin  by  putting,  as  it  were,  the 
problem  into  shape,  and  by  enumerating  certain  con- 
ditions which,  though  they  may  not  be  absolutely 
fulfilled  in  nature,  are  often  so  very  nearly  fulfilled 
that  we  make  no  appreciable  error  by  supposing  them 
to  be  so. 

Let  us  suppose  that  a  myriad  bodies  of  various 
sizes,  shapes,  materials  and  masses,  are  launched  in 
space  in  any  order  whatever,  at  any  distances  from  each 
other,  and  that  they  are  started  with  very  different 


A   PROBLEM   STATED.  215 

movements.  Some  may  be  going  very  fast,  some  going 
slowly,  or  not  at  all ;  some  may  be  moving  up  or  down 
or  to  the  right  or  to  the  left — there  may  be,  in  fact, 
everjr  variety  in  their  distances  and  their  velocities, 
and  in  the  directions  in  which  they  are  started. 

We  assume  that  each  pair  of  masses  attract  each 
other  by  the  well-known  law  of  gravitation,  which 
expresses  that  the  force  between  any  two  bodies  is 
proportional  directly  to  the  product  of  their  masses 
and  inversely  to  the  square  of  their  distance.  We 
have  one  further  supposition  to  make,  and  it  is  an 
important  one.  We  shall  assume  that  though  each 
one  of  the  bodies  which  we  are  considering  is  affecting 
all  the  others,  and  is  in  turn  affected  by  them,  yet 
that  they  are  subjected  to  no  appreciable  disturbing 
influence  from  other  bodies  not  included  in  the  system 
to  which  they  belong.  This  may  seem  at  first  to  make 
the  problem  we  are  about  to  consider  a  purely  imagi- 
nary one,  such  as  could  only  be  applicable  to  systems 
different  from  those  which  are  actually  presented  to 
us  in  nature.  It  must  be  admitted  that  the  condition 
we  have  inferred  can  only  be  approximately  fulfilled. 
But  a  little  consideration  will  show  that  the  supposi- 
tion is  not  an  unreasonable  one.  Take,  for  instance, 
the  solar  system,  consisting  of  the  sun,  the  planets, 
and  their  satellites.  Every  one  of  these  bodies  attracts 
every  other  body,  and  the  movement  of  each  of  the 
bodies  is  produced  by  the  joint  effects  of  the  forces 
exerted  upon  it  by  all  the  others.  Assuredly  this 
gives  a  problem  quite  difficult  enough  for  all  the 
resources  that  are  at  our  command.  But  in  such 
investigations  we  omit  altogether  the  influence  of  the 
stars.  Sirius,  for  example,  does  exercise  some  attrac- 


216  THE    EARTH'S    BEGINNING. 

notion  the  bodies  of  our  system,  but  owing  to  its 
enormous  distance,  in  comparison  with  the  distances 
in  our  solar  system,  the  effect  of  the  disturbance  of 
Sirius  on  the  relative  movements  of  the  planets  is 
wholly  inappreciable.  Indeed,  we  may  add  that  the 
disturbances  in  the  solar  system  produced  by  all  the 
stars,  even  including  the  myriads  of  the  Milky  Way, 
are  absolutely  negligible.  ,  The  movements  in  our 
solar  system,  so  far  as  our  observations  reveal  them, 
are  performed  precisely  as  if  all  bodies  of  the  universe 
foreign  to  the  solar  system  were  non-existent.  This 
consideration  shows  that  in  the  problem  we  are  now 
to  consider,  we  are  introducing  no  unreasonable  element 
when  we  premise  that  the  system  whose  movements 
we  are  to  investigate  is  to  be  regarded  as  free  from 
appreciable  disturbance  by  any  foreign  influence. 

To  follow  the  fortunes  of  a  system  of  bodies,  large 
or  small,  starting  under  any  arbitrary  conditions  at 
the  commencement,  and  then  abandoned  to  their 
mutual  attractions,  is  a  problem  for  the  mathematician. 
It  certainly  presents  to  him  questions  of  very  great 
difficulty,  and  many  of  these  he  has  to  confess  are 
insoluble  ;  there  are,  however,  certain  important  laws 
which  must  be  obeyed  in  all  the  vicissitudes  of  the 
motion.  There  are  certain  theorems  known  to  the 
mathematician  which  apply  to  such  a  system,  and  it 
is  these  theorems  which  afford  us  most  interesting  and 
instructive  information.  I  am  well  aware  that  the 
subject  upon  which  I  am  about  to  enter  is  not  a 
very  easy  one,  but  its  importance  is  such  that  I  must 
make  the  effort  to  explain  it. 

Let  me  commence  by  describing  what  is  meant 
when  we  speak  of  the  energy  of  a  system.  Take,  first, 


THE    ENERGY    OF   A    SYSTEM.  217 

the  case  of  merely  two  bodies,  and  let  us  suppose  that 
they  were  initially  at  rest.  The  energy  of  a  system 
of  this  very  simple  type  is  represented  by  the  quantity 
of  work  which  could  be  done  by  allowing  these  two 
bodies  to  come  together.  If,  instead  of  being  in  the 
beginning  simply  at  rest,  the  bodies  had  each  been  in 
motion,  the  energy  of  the  system  would  be  correspond- 
ingly greater.  The  energy  of  a  moving  body,  or  its 
capacity  of  doing  work  in  virtue  of  its  movement,  is 
proportional  jointly  to  its  mass  and  to  the  square  of 
its  velocity.  The  energy  of  the  two  moving  bodies  will 
therefore  be  represented  by  three  parts ;  first,  there 
will  be  that  due  to  their  distance  apart;  secondly, 
there  will  be  that  due  to  the  velocity  of  one  of  them ; 
and,  thirdly,  there  is  that  due  to  the  velocity  of  the 
other.  In  the  case  of  a  number  of  bodies,  the  energy 
will  consist  in  the  first  place  of  a  part  which  is  due 
to  the  separation  of  the  bodies,  and  measured  by  the 
quantity  of  work  that  would  be  produced  if,  in  obedience 
to  their  mutual  attraction,  all  the  bodies  were  allowed 
to  come  together  into  one  mass.  In  the  second  place, 
the  bodies  are  to  be  supposed  to  have  been  originally 
started  with  certain  velocities,  and  the  energy  of  each 
of  the  bodies,  in  virtue  of  its  motion,  is  to  be  measured 
by  the  product  of  one -half  its  mass  into  the  square  of 
its  velocity.  The  total  energy  of  the  system  consists, 
therefore,  of  the  sum  of  the  parts  due  to  the  velocities 
of  the  bodies,  and  that  which  is  due  to  their  mutual 
separation. 

If  the  bodies  could  really  be  perfectly  rigid,  unyield- 
ing masses,  so  that  they  have  no  movements  analogous 
to  tides,  and  if  their  movements  be  such  that  collisions 
will  not  take  place  among  them,  then  the  laws  of 


218  THE   EARTH'S   BEGINNING. 

mechanics  tell  us  that  the  quantity  of  energy  in  that 
system  will  remain  for  ever  unaltered.  The  velocities  of 
the  particles  may  vary,  and  the  mutual  distances  of  the 
particles  may  vary,  but  those  variations  will  be  always 
conducted,  subject  to  the  fundamental  condition  that  if 
we  multiply  the  square  of  the  velocity  of  each  body  by 
one-half  its  mass,  and  add  all  those  quantities  together, 
and  if  we  increase  the  sum  thus  obtained  by  the  quantity 
of  energy  equivalent  to  the  separation  of  the  particles, 
the  total  amount  thus  obtained  is  constant.  This 
is  the  fundamental  law  of  mechanics  known  as  the 
conservation  of  energy. 

For  such  material  systems  as  the  universe  presents  to 
us,  the  conservation  of  energy,  in  the  sense  in  which 
1  have  here  expressed  it,  will  not  be  maintained ;  for  the 
necessary  conditions  cannot  be  fulfilled.  Let  us  suppose 
that  the  incessant  movements  of  the  bodies  in  the  system, 
rushing  about  under  the  influence  of  their  mutual 
attractions,  has  at  last  been  productive  of  a  collision 
between  two  of  the  bodies.  We  have  already  explained 
in  Chapter  VI.  how  in  the  collision  of  two  masses  the 
energy  which  they  possess  in  virtue  of  their  movements 
may  be  to  a  large  extent  transformed  into  heat ;  there  is 
consequently  an  immediate  increase  in  the  temperature 
of  the  bodies  concerned,  and  then  follows  the  operation 
of  that  fundamental  law  of  heat,  by  which  the  excess  of 
heat  so  arising  will  be  radiated  away.  Some  of  it  will, 
no  doubt,  be  intercepted  by  falling  on  other  bodies  in 
the  system,  and  the  amount  that  might  be  thus  possibly 
retained  would,  of  course,  not  be  lost  to  the  system 
The  bodies  of  the  solar  system  at  least  are  so  widely 
scattered,  that  the  greater  part  of  the  heat  would  cer- 
tainly escape  into  space,  and  the  corresponding  quantity 


THE    LOSS    OF   ENERGY.  219 

of  energy  would  be  totally  lost  to  the  system.  We 
may  generally  assume  that  a  collision  among  the  bodies 
would  be  most  certainly  productive  of  a  loss  of  energy 
from  the  system. 

No  doubt  collisions  can  hardly  be  expected  to  occur 
in  a  system  consisting  of  large,  isolated  bodies  like  the 
planets.  Even  in  any  system  of  solid  bodies  collisions 
may  be  presumed  to  be  infrequent  in  comparison  with 
the  numbers  of  the  bodies.  But  if,  instead  of  a  system  of 
few  bodies  of  large  mass,  we  have  a  gas  or  nebula  com- 
posed of  innumerable  atoms  or  molecules,  the  collisions 
would  be  by  no  means  infrequent,  and  every  collision,  in 
so  far  as  it  led  to  the  production  of  heat,  would  be 
productive  of  loss  of  energy  by  radiation  from  the 
system. 

It  should  also  be  added  that,  even  independently  of 
actual  collisions,  there  is,  and  must  be,  loss  of  energy  in 
the  system  from  other  causes.  There  are  no  absolutely 
rigid  bodies  known  in  nature,  for  the  hardest  mineral  or 
the  toughest  steel  must  yield  to  some  extent  when  large 
forces  are  applied  to  it,  and  as  the  bodies  in  the  system 
are  not  mere  points  or  particles  of  inconsiderable  dimen- 
sions, they  will  experience  stresses  something  like  those 
to  which  our  earth  is  subjected  in  that  action  of  the 
moon  and  sun  which  produces  the  tides.  In  conse- 
quence of  the  influences  of  each  body  on  the  rest,  there 
will  be  certain  relative  changes  in  the  parts  of  each 
body ;  there  will  be,  as  it  were,  tidal  movements  in  their 
liquid  parts  and  even  in  their  solid  substance.  These 
tides  will  produce  friction,  and  this  will  produce  heat. 
This  heat  will  be  radiated  from  the  system,  but  the  heat 
radiated  corresponds  to  a  certain  amount  of  energy  ;  the 
energy  is  therefore  lost  to  the  system,  so  that  even  with- 


220  THE   EARTH'S    BEGINNING. 

out  actual  collisions  we  still  find  that  energy  must  be 
gradually  lost  to  the  system. 

Thus  we  have  been  conducted  to  an  important 
conclusion,  which  may  be  stated  in  the  following  way. 
Let  there  be  any  system  of  bodies,  subject  to  their 
mutual  attractions,  and  sufficiently  isolated  from  the 
disturbing  influence  of  all  bodies  which  do  not  belong  to 
the  system,  then  the  original  energy  with  which  that 
system  is  started  must  be  undergoing  a  continual  decline. 
It  must  at  least  decline  until  such  a  condition  of  the 
system  has  been  reached  that  collisions  are  no  longer 
possible  and  that  tidal  influences  have  ceased.  These 
conditions  might  be  fulfilled  if  all  the  bodies  of  the 
system  coalesced  into  a  single  mass. 

As  illustrations  of  the  systems  Ave  are  now  con- 
sidering, we  may  take  the  sun  and  planets  as  a  whole. 
A  spiral  nebula  is  a  system  in  the  present  sense,  while 
the  grandest  illustration  of  all  is  provided  by  the  Milky 
Way. 

It  will  be  noted  that  we  may  have  a  system  which 
is  isolated  so  far  as  our  present  argument  is  concerned, 
even  while  it  forms  a  part  of  another  system  of  a  higher 
order  of  magnitude.  For  instance,  Saturn  with  his 
rings  and  satellites  is  sufficiently  isolated  from  the  rest 
of  the  solar  system  and  the  rest  of  the  universe,  to 
enable  us  to  trace  the  consequences  of  the  gradual 
decline  of  energy  in  his  attendant  system.  The  solar 
system  in  which  Saturn  appears  merely  as  a  unit,  is 
itself  sufficiently  isolated  from  the  stars  in  the  Milky 
Way  to  permit  us  to  study  the  decline  of  energy  in  the 
solar  system,  without  considering  the  action  of  those 
stars. 

This   general   law  of  the   decline   of  energy  in  an 


NOTHING    IS    AT    REST.  221 

isolated  system,  is  supplemented  by  another  law  often 
known  as  the  conservation  of  moment  of  momentum. 
It  may  at  first  seem  difficult  to  grasp  the  notion  which 
this  law  involves.  The  effort  is,  however,  worth  making, 
for  the  law  in  question  is  of  fundamental  importance  in 
the  study  of  the  mechanics  of  the  universe.  In  the 
Appendix  will  be  found  an  investigation  by  elementary 
geometry  of  the  important  mechanical  principles  which 
are  involved  in  this  subject. 

Whatever  may  have  been  the  origin  of  the  primseval 
nebula,  and  whatever  may  have  been  the  forces  con- 
cerned in  its  production  we  may  feel  confident  that  it 
was  not  originally  at  rest.  We  do  not  indeed  know  any 
object  which  is  at  rest.  Not  one  of  the  heavenly  bodies 
is  at  rest;  nothing  on  earth  is  at  rest,  for  even  the 
molecules  of  rigid  matter  are  in  rapid  motion.  Rest 
seems  unknown  in  the  universe.  It  would  be,  therefore, 
infinitely  improbable  that  a  primaeval  nebula,  whatever 
may  have  been  the  agency  by  which  it  was  started  on 
that  career  which  we  are  considering,  was  initially  in  a 
condition  of  absolute  rest.  We  assume  without  hesi- 
tation that  the  nebula  was  to  some  extent  in  motion,  and 
we  may  feel  assured  that  the  motions  were  of  a  highly 
complicated  description.  It  is  fortunate  for  us  that  our 
argument  does  not  require  us  .to  know  the  precise 
character  of  the  movements,  as  such  knowledge  would 
obviously  be  quite  unattainable.  We  can,  however, 
invoke  the  laws  of  mechanics  as  an  unerring  guide. 
They  will  tell  us  not  indeed  everything  about  those 
motions,  but  they  will  set  forth  certain  characteristics 
which  the  movements  must  have  had,  and  these 
characteristics  suffice  for  our  argument. 

To  illustrate  the  important  principle  on  which  we 


222  THE    EARTH'S    BEGINNING. 

are  now  entering  I  must  mention  the  famous  problem  of 
three  bodies  which  has  engaged  the  attention  of  the 
greatest  mathematicians.  Let  there  be  a  body  A,  and 
another  B,  and  another  C.  We  shall  suppose  that 
these  bodies  are  so  small  that  they  may  be  regarded 
merely  as  points  in  comparison  with  the  distances  by 
which  they  are  separated.  We  shall  suppose  that  they 
are  all  moving  in  the  same  plane,  and  we  shall  suppose 
that  each  of  them  attracts  the  others,  but  that  except 
these  attractions  there  are  no  other  forces  in  the  system. 
To  discover  all  about  the  motions  of  these  bodies  is  so 
difficult  a  problem  that  mathematicians  have  never 
been  able  to  solve  it.  But  though  we  are  not  able  to 
solve  the  problem  completely,  we  can  learn  something 
with  regard  to  it. 

We  represent  by  arrows  in  Fig.  36  the  directions  in 
which  A,  B,  and  C  are  moving  at  the  moment.  We  choose 
any  point  O  in  the  plane,  and  for  simplicity  we  have  so 
drawn  the  figure  that  A,  B,  and  C  are  forces  tending 
to  turn  round  0  in  the  same  direction.  The  velocity  of 
a  body  multiplied  into  its  mass  is  termed  the  momentum 
of  the  body.  Draw  the  perpendicular  from  O  to  the 
direction  .  in  which  the  body  A  is  moving,  then  the 
product  of  this  perpendicular  and  the  momentum  of  A 
is  called  the  moment  of  momentum  of  A  around  0.  In 
like  manner  we  form  the  moment  of  momentum  of  B 
and  C,  and  if  we  add  them  together  we  obtain  the  total 
moment  of  momentum  of  the  system. 

We  can  now  give  expression  to  a  great  discovery 
which  mathematicians  have  made.  No  matter  how 
complicated  may  be  the  movements  of  A,  B,  and  C ;  no 
matter  to  what  extent  these  particles  approximate  or 
how  widely  they  separate ;  no  matter  what  changes  may 


MOMENT    OF   MOMENTUM. 


223 


occur  in  their  velocities,  or  even  what  actual  collision 
may  take  place,  the  sum  of  the  moments  of  momentum 
must  remain  for  ever  unaltered.  This  most  important 
principle  in  dynamics  is  known  as  the  conservation  of 
moment  of  momentum. 

Though  I  have  only  mentioned  three  particles,  yet 
the  same  principle  will  be  true  for  any  number.  If  it 
should  happen  that 
any  of  them  are  turn- 
ing round  0  in  the  op- 
posite direction,  then 
their  moments  of  mo- 
mentum are  to  be 
taken  as  negative. 
In  this  case  we  add 
the  moments  tending 
in  one  direction  to- 
gether ;  and  then  sub- 
tract all  the  opposite 
moments.  The  re- 
mainder is  the  quan- 
tity which  remains 
constant. 

We  may  state  this  principle  in  a  somewhat  different 
manner  as  follows :  Let  us  consider  a  multitude  of 
particles  in  a  plane ;  let  them  be  severally  started  in  any 
directions  in  the  plane,  and  then  be  abandoned  to 
their  mutual  attractions,  it  being  understood  that  there 
are  no  forces  produced  by  bodies  external  to  the  system ; 
if  we  then  choose  any  point  in  the  plane,  and  measure 
the  areas  described  round  that  point  by  the  several 
moving  bodies  in  one  second,  and  if  we  multiply  each  of, 
those  areas  by  the  mass  of  the  corresponding  body,  then, 


Fig.    36.  -  To    ILLUSTRATE    MOMENT    OF 

MOMENTUM. 


224  THE   EARTH'S    BEGINNING. 

if  all  the  bodies  are  moving  in  the  same  direction  round 
the  point,  the  sum  of  the  quantities  so  obtained  is 
constant.  It  will  be  the  same  a  hundred  or  a  thousand 
years  hence  as  it  is  at  the  present  moment,  or  as  it  was 
a  hundred  or  a  thousand  years  ago.  If  any  of  the  par- 
ticles had  been  turning  round  the  point  in  the  opposite 
direction,  then  the  products  belonging  to  such  particles 
are  to  be  subtracted  from  the  others  instead  of  added. 

We  have  now  to  express  in  a  still  more  general  manner 
the  important  principle  that  is  here  involved.  Let  us 
consider  any  system  of  attracting  particles,  no  matter 
what  their  masses  or  whether  their  movements  be 
restricted  to  a  plane  or  not.  Let  us  start  them  into 
motion  in  any  directions  and  with  any  initial  velocities, 
and  then  abandon  them  to  the  influence  of  their  mutual 
attractions,  withholding  at  the  same  time  the  inter- 
ference of  any  forces  from  bodies  exterior  to  the 
system.  Draw  any  plane  whatever,  and  let  fall  perpen- 
diculars upon  this  plane  from  the  different  particles 
of  the  system.  It  will  be  obvious  that  as  the  particles 
move  the  feet  of  the  perpendiculars  must  move  in 
correspondence  with  the  particles  from  which  the 
perpendiculars  were  let  fall.  We  may  regard  the 
foot  of  every  perpendicular  as  the  actual  position  of 
a  moving  point,  and  it  can  be  proved  that  if  the  mass  of 
each  particle  be  multiplied  into  the  area  which  the  foot 
of  its  perpendicular  describes  in  a  second  round  any 
point  in  the  plane,  and  then  be  added  to  the  similar 
products  from  all  the  other  particles,  only  observing  the 
proper  precautions  as  to  sign,  the  sum  will  remain 
constant,  i.e.,  in  any  other  second  the  total  quantity 
arrived  at  will  be  exactly  the  same.  This  is  a  general 
law  of  dynamics.  It  is  not  a  law  of  merely  approximate 


THE    "PRINCIPAL    PLANE."  225 

truth,  it  is  a  law  true  with  absolute  accuracy  during 
unlimited  periods  of  time. 

The  actual  value  of  the  constant  will  depend  both 
on  the  system  and  on  the  plane.  For  a  given  system 
the  constant  will  differ  for  the  different  planes  which 
may  be  drawn,  and  there  will  be  some  planes  in  which 
that  sum  will  be  zero.  In  other  words,  in  those  planes 
the  areas  described  by  the  feet  of  the  perpendiculars, 
multiplied  by  the  masses  of  the  particles  which  are 
moving  in  one  way,  will  be  precisely  equal  to  the 
similar  sum  obtained  from  the  particles  moving  in  the 
opposite  direction. 

But  among  all  possible  planes  there  is  one  of 
special  significance  in  its  relation  to  the  system.  It 
is  called  the  "principal  plane,"  and  it  is  characterised 
by  the  fact  that  the  sum  (with  due  attention  to 
sign)  of  the  areas  described  each  second  by  the  feet 
of  the  perpendiculars,  multiplied  into  the  masses  of  the 
corresponding  particle,  is  greater  than  the  like  magni- 
tude for  any  other  plane,  and  is  thus  a  maximum. 
For  all  planes  parallel  to  this  principal  plane,  the  result 
will  be,  of  course,  the  same ;  it  is  the  direction  of  the 
plane  and  not  its  absolute  situation  that  is  material. 
We  thus  see  that  while  this  remarkable  quantity  is 
constant  in  any  plane,  for  all  time,  yet  the  actual 
value  of  that  constant  depends  upon  the  aspect  of 
the  plane ;  for  some  planes  it  is  zero,  for  others  the 
constant  has  intermediate  values,  and  there  is  one 
plane  for  which  the  constant  is  a  maximum.  This  is 
the  principal  plane,  and  a  knowledge  of  it  is  of  vital 
importance  in  endeavouring  to  understand  the  nebular 
theory.  Nor  are  the  principles  under  consideration 
limited  only  to  a  system  consisting  of  sun  and  planets 


226  THE   EARTH'S    BEGINNING. 

they  apply,  with  suitable  modifications,  to  many  other 
celestial  systems  as  well. 

The  instructive  character  of  this  dynamical  principle 
will  be  seen  when  we  deduce  its  consequences.  The 
term  "  moment  of  momentum "  of  a  particle,  with 
reference  to  a  certain  point  in  a  plane,  expresses  double 
the  product  of  the  rate  at  which  the  area  is  described  by 
the  foot  of  the  perpendicular  to  this  plane,  multiplied  by 
the  mass  of  the  particle.  The  moment  of  momentum  of 
the  system,  with  reference  to  the  principal  plane,  is  a 
maximum  in  comparison  with  all  other  planes ;  that 
moment  of  momentum  retains  precisely  the  same  value 
throughout  all  time,  from  the  first  instant  the  system 
was  started  onwards.  And  it  retains  this  value,  no 
matter  what  changes  or  disturbances  may  happen  in 
the  system,  provided  only  that  the  influence  of  ex- 
ternal forces  is  withheld.  Subject  to  this  condition,  the 
transformations  of  the  system  may  be  any  whatever. 
The  several  bodies  may  be  forced  into  wide  changes 
of  their  orbits,  so  that  there  may  even  be  collisions 
among  them ;  yet,  notwithstanding  those  collisions,  and 
notwithstanding  the  violent  alterations  which  may  be 
thus  produced  in  the  movements  of  the  bodies,  the 
moment  of  momentum  will  not  alter.  No  matter  what 
tides  may  be  produced,  even  if  those  tides  be  so 
great  as  to  produce  disruption  in  the  masses  and 
force  the  orbits  to  change  their  character  radically,  yet 
the  moment  of  momentum  will  be  conserved  without 
alteration. 

It  is  essential  to  notice  the  fundamental  difference 
between  the  principle  which  has  been  called  the  con- 
servation of  energy  in  the  system,  and  the  conservation 
of  moment  of  momentum.  We  have  pointed  out  that 


AN  .IMPROBABLE    SYSTEM.  227 

when  collisions  take  place,  part  of  the  energy  due  to 
motion  is  transformed  into  heat,  and  energy  in  that 
form  admits  of  radiation  through  space,  and  thus 
becomes  lost  to  the  system,  with  the  result  that  the 
total  energy  declines.  Even  without  actual  collision, 
we  have  shown  how  certain  effects  of  tides,  or  other 
consequences  of  friction,  necessarily  involve  the  squan- 
dering of  energy  with  which  the  system,  was  originally 
endowed.  A  system  started  with  a  certain  endowment 
of  energy  may  conserve  that  energy  indefinitely,  if 
all  such  actions  as  collisions  or  frictions  are  absent. 
If  collisions  or  frictions  are  present  the  system  will 
gradually  dissipate  energy.  Our  interpretation  of  the 
future  of  such  a  system  must  always  take  account  of 
this  fundamental  fact. 

It  is,  of  course,  conceivable  that  the  moment  of 
momentum  with  which  a  system  was  originally  endowed 
might  have  happened  to  be  zero.  A  system  of  particles 
could  be  so  constructed  and  so  started  on  their  move- 
ments that  their  moment  of  momentum  with  regard  to 
a  certain  plane  should  be  zero.  It  might  happen  that  the 
moment  of  momentum  of  the  system  with  regard  to  a 
second  plane,  perpendicular  to  the  former  one,  should 
be  also  zero;  and,  finally,  that  the  moment  of  mo- 
mentum of  the  system  with  regard  to  a  third  plane  per- 
pendicular to  each  of  the  other  two,  should  be  also  zero. 
If  these  three  conditions  were  found  to  prevail  at 
the  commencement,  they  would  prevail  throughout 
the  movement,  and,  more  generally  still,  we  may 
state  that  in  such  circumstances  the  moment  of 
momentum  of  the  system  would  be  zero  about  any 
plane  whatever.  There  would  be  no  principal  plane 
in  such  a  system.  We  thus  note  that  though  it  is 


228  THE   EARTH'S    BEGINNING. 

inconceivable  that  a  group  of  mutually  attracting 
bodies  should  be  started  into  movement  without  a 
suitable  endowment  of  energy,  it  is  yet  quite  conceiv- 
able that  a  system  could  be  started  without  having 
any  moment  of  momentum.  And  if  at  the  beginning 
the  system  had  no  moment  of  momentum,  then  no 
matter  what  may  be  the  future  vicissitudes  of  its 
motion,  no  moment  of  momentum  can  ever  be  acquired 
by  it  to  all  eternity,  so  long  as  the  interference  of 
external  forces  is  excluded. 

But  having  said  this  much  as  to  the  conceivability 
of  the  initiation  of  a  system  with  no  moment  of  momen- 
tum, we  now  hasten  to  add  that,  so  far  as  Nature  is 
actually  concerned,  this  bare  possibility  may  be  set 
aside  as  one  which  is  infinitely  improbable.  Nature 
does  not  do  things  which  are  infinitely  improbable,  and, 
therefore,  we  may  affirm  that  all  material  systems,  with 
which  we  shall  have  to  deal,  do  possess  moment  of 
momentum.  However  the  system  may  have  originated, 
whatever  may  have  been  the  actions  of  forces  by  which 
it  was  brought  into  being,  we  may  feel  assured  that 
the  system  received  at  its  initiation  some  endowment  of 
moment  of  momentum,  as  well  as  of  energy.  Hence  we 
may  conclude  that  every  such  system  as  is  presented  to 
us  in  the  infinite  variety  of  Nature,  must  stand  in 
intimate  relation  to  some  particular  plane,  being  that 
which  is  known  as  the  principal  plane  of  moment  of 
momentum.  In  our  effort  to  interpret  Nature,  the 
physical  importance  of  this  fact  can  hardly  be  over- 
estimated. 

In  a  future  chapter  we  shall  make  some  attempt 
to  sketch  the  natural  operations  by  which  individual 
systems  have  been  started  on  their  careers.  Postponing, 


DRAWING   INFERENCES.  229 

then,  such  questions,  we  propose  to  deal  now  with  the 
phenomena  which  the  principles  of  dynamics  declare 
must  accompany  the  evolution  of  a  system  under  the 
action  of  the  exclusive  attraction  of  the  various  parts 
of  that  system  for  each  other.  The  system  commences 
its  career  with  a  certain  endowment  of  energy,  with  a 
certain  endowment  of  moment  of  momentum,  and  with 
a  certain  principal  plane  to  which  that  moment  of 
momentum  is  specially  related.  In  the  course  of  the 
evolution  through  which,  in  myriads  of  ages,  the  system 
is  destined  to  pass,  the  energy  that  it  contains  will 
undergo  vast  loss  by  dissipation.  On  the  other  hand, 
the  moment  of  momentum  will  never  vary,  and  the 
position  of  the  principal  plane  will  remain  the  same  for 
all  time.  We  have  to  consider  what  features,  connected 
with  the  evolution,  may  be  attributed  to  the  operation  of 
these  dynamical  laws.  We  have,  in  fact,  to  deduce  the 
consequences  which  seem  to  follow  from  the  fact  that, 
in  consequence  of  collisions,  and  in  consequence  of 
friction,  an  isolated  system  in  space  must  gradually  part 
with  its  initial  store  of  energy,  but  that,  notwithstanding 
any  collisions  and  any  friction,  the  total  moment  of 
momentum  of  the  system  suffers  no  abatement. 

As  the  system  advances  in  development,  we  have 
to  deal  with  a  gradual  decline  in  the  ratio  of  the  original 
store  of  energy  to  the  original  store  of  moment  of 
momentum.  And  hence  we  must  expect  that  a  system 
will  ultimately  tend  towards  a  form  in  which,  while 
preserving  its  moment  of  momentum,  it  shall  do  so  with 
such  a  distribution  of  the  bodies  of  which  it  consists 
as  shall  be  compatible  with  a  diminishing  quantity  of 
energy.  It  is  not  hard  to  see  that  in  the  course  of  ages 
this  tends,  as  one  consequence,  to  make  the  movements 
16 


230  THE    EAETH'S    BEGINNING. 

of  each  of  the  bodies  in  the  system  ultimately  approxi- 
mate to  movements  in  a  plane. 

Let  us,  for  simplicity,  begin  with  the  case  of  three 
attracting  particles,  A,  B  and  C.  Let  B  be  started  in 
any  direction  in  the  plane  L,  and  let  A  be  started  in  an 
orbit  round  it,  and  in  the  same  plane  L.  Now  let  C  be 
started  into  motion,  in  any  direction,  from  some  point 
also  in  L.  It  is  certain  that  the  sum  of  the  areas 
projected  parallel  to  any  plane,  which  are  described  in  a 
second  by  these  three  bodies,  must  be  constant,  each 
of  the  areas  being,  as  usual,  multiplied  by  the  mass  of 
the  corresponding  body.  Let  us  specially  consider  the 
plane  L  in  which  the  motions  of  A  and  B  already  lie. 
It  is  on  this  plane  that  the  area  described  by  C  has  to  be 
projected.  The  essential  point  now  to  remember  is  that 
the  projected  area  is  less  than  the  actual  area.  It  is 
plain  that  if  C  has  to  describe  a  certain  projected  area  in 
a  certain  time,  the  velocity  with  which  C  has  to  move 
must  be  greater  when  C  starts  off  at  an  inclination  to 
the  plane  than  would  have  been  necessary  if  C  had 
started  in  the  plane,  other  things  being  the  same.  Thus 
we  see  that,  if  the  three  bodies  were  all  moving  in  the 
same  plane,  they  could,  speaking  generally,  maintain 
more  easily  the  requisite  description  of  areas,  that  is,  the 
requisite  moment  of  momentum  with  smaller  velocities 
than  if  they  were  moving  in  directions  which  were  not 
so  regulated ;  that  is  to  say,  the  moment  of  momentum 
can  be  kept  up  with  less  energy  when  the  particles  move 
in  the  same  plane. 

In  a  more  general  manner  we  see  that  any  system 
in  which  the  bodies  are  moving  in  the  same  plane 
will,  for  equal  moment  of  momentum,  require  less 
energy  than  it  would  have  done  had  the  bodies  been 


SATURN'S    RING.  231 

moving  in  directions  which  were  not  limited  to  a  plane. 
Thus  we  are  led  to  the  conclusion  that  the  ultimate 
result  of  the  collisions  and  the  friction  and  the  tides, 
which  are  caused  by  the  action  of  one  particle  on 
another,  is  to  make  the  movements  tend  towards  the 
same  plane. 

In  this  dynamical  principle  we  have  in  all  pro- 
bability a  physical  explanation  of  that  remarkable 
characteristic  of  celestial  movements  to  which  we  have 
referred.  The  solar  system  possesses  less  energy  in 
proportion  to  its  moment  of  momentum  than  it  would 
require  to  have  if  the  orbits  of  the  important  planets,- 
instead  of  lying  practically  in  the  same  plane,  were 
inclined  at  various  angles.  Whatever  may  have  been 
the  original  disposition  of  the  materials  forming  the 
solar  system,  they  must  once  have  contained  much 
more  energy  than  they  have  at  present.  The  moment 
of  momentum  in  the  principal  plane,  at  the  beginning, 
was  not,  however,  different  from  the  moment  of 
momentum  that  the  system  now  possesses.  As  the 
energy  of  the  system  gradually  declined,  the  system 
has  gradually  been  compelled  to  adjust  itself  in  such 
a  manner  that,  with  the  reduced  quantity  of  energy, 
the  requisite  moment  of  momentum  shall  still  be 
preserved.  This  is  the  reason  why,  in  the  course  of 
the  myriads  of  ages  during  which  the  solar  system 
has  been  acquiring  its  present  form,  the  movements 
have  gradually  become  nearly  conformed  to  a  plane. 

The  operation  of  the  principle,  now  before  us,  may 
be  seen  in  a  striking  manner  in  Saturn's  ring.  (Fig.  37.) 
The  particles  constituting  this  exquisite  object,  so  far 
as  observations  have  revealed  them,  seem  to  present 
to  us  an  almost  absolutely  plane  movement.  The  fact 


232  THE    EARTH'S    BEGINNING. 

that  the  movements  of  the  constituents  of  Saturn's 
ring  lie  in  a  plane  is  doubtless  to  be  accounted  for 
by  the  operation  of  the  fundamental  dynamical  prin- 
ciple to  which  we  have  referred.  Saturn,  in  its  great 
motion  round  the  luminary,  is,  of  course,  controlled 
by  the  sun,  yet  the  system  attached  to  Saturn  is  so 
close  to  that  globe  as  to  be  attracted  by  the  sun  in 
a  manner  which  need  not  here  be  distinguished  from 
the  solar  attraction  on  Saturn  itself.  It  follows  that 
the  differential  action,  so  to  speak,  of  the  sun  on  Saturn, 
and  on  the  myriad  objects  which  constitute  its  ring,  may- 
be disregarded.  We  are  therefore  entitled,  as  already 
mentioned,  to  view  Saturn  and  its  system  as  an 
isolated  group,  not  acted  upon  by  any  forces  exterior 
to  the  system.  It  is  therefore  subject  to  the  laws 
which  declare  that,  though  the  energy  declines,  the 
moment  of  momentum  is  to  remain  unaltered.  This 
it  is  which  has  apparently  caused  the  extreme  flatness 
of  Saturn's  ring.  The  energy  of  the  rotation  of  that 
system  has  been  expended  until  it  might  seem  that 
no  more  energy  has  been  left  than  just  suffices  to 
preserve  the  unalterable  moment  of  momentum,  under 
the  most  economical  conditions,  so  far  as  energy  is 
concerned. 

Let  us  suppose  that  one  of  the  innumerable  myriads 
of  particles  which  constitute  the  ring  of  Saturn  were 
to  forsake  the  plane  in  which  it  now  revolves,  and 
move  in  an  orbit  inclined  to  the  present  plane.  We 
shall  suppose  that  the  original  track  of  the  orbit  was 
a  circle,  and  we  shall  assume  that  in  the  new  plane 
to  which  the  motion  is  transferred  the  motion  is  also 
circular,  That  particle  will  have  still  to  do  its  share 
of  preserving  the  requisite  total  moment  of  momentum, 


a 


£ 

I 

*>i 

CO 

t® 


234  THE   EARTH'S    BEGINNING. 

for  we  are  to  suppose  that  each  of  the  other  particles 
remains  unaltered  in  its  pace  and  in  the  other  circum- 
stances of  its  motion.  The  aberrant  particle  will  de- 
scribe, in  a  second,  an  area  which,  for  the  purpose  of 
the  present  calculation,  must  be  projected  upon  the 
plane  containing  the  other  particles.  The  area,  when 
projected,  must  still  be  as  large  as  the  area  that  the 
particle  would  have  described  -if  it  had  remained  in 
the  plane.  It  is  therefore  necessary  that  the  area  swept 
over  by  the  particle  in  the  inclined  plane,  in  one  second, 
shall  be  greater  than  the  area  which  sufficed  in  the 
original  plane.  This  requires  the  circle  in  which  the 
particle  revolves  to  be  enlarged,  and  this  necessitates 
that  its  energy  should  be  increased.  In  other  words, 
while  the  moment  of  momentum  was  no  greater  than 
before,  the  energy  of  the  system  would  have  to  be 
greater.  We  thus  see  that  inasmuch  as  the  particles 
forming  the  rings  of  Saturn  move  in  circles  in  the  same 
plane,  they  require  a  smaller  amount  of  energy  in  the 
system  to  preserve  the  requisite  moment  of  momentum 
than  would  be  required  if  they  moved  in  circular  orbits 
which  were  not  in  the  same  plane.  In  such  a  system 
as  Saturn's  ring,  in  which  the  particles  are  excessively 
numerous  and  excessively  close  together,  it  may  be 
presumed  that  there  may  once  have  been  sufficient 
collisions  and  frictions  among  the  particles  to  cause 
the  exhaustion  of  energy  to  the  lowest  point  at  which 
the  moment  of  momentum  would  be  sustained.  In 
the  course  of  ages  this  has  been  accomplished  by 
the  remarkable  adjustment  of  the  movements  to  that 
plane  in  which  we  now  find  them. 

The  importance  of  this  subject  is  so  great  that  we 
shall  present  the  matter  in  a  somewhat  different  manner 


ENERGY   AND    DISTANCE.  235 

as  follows :  We  shall  simplify  the  matter  by  regarding 
the  orbits  of  the  planets  or  other  bodies  as  circles. 
The  fact  that  these  orbits  are  ellipses,  which  are, 
however,  very  nearly  circles,  will  not  appreciably 
affect  the  argument. 

Let  us,  then,  suppose  a  single  planet  revolving 
round  a  fixed  sun,  in  the  centre.  The  energy  of  this 
system  has  two  parts.  There  is  first  the  energy  due  to 
the  velocity  of  the  planet,  and  this  is  found  by  taking 
half  the  product  of  the  mass  of  the  planet  and  the 
square  of  its  velocity.  The  second  part  of  the  energy 
depends,  as  we  have  already  explained,  on  the  distance 
of  the  planet  from  the  sun.  The  planet  possesses  energy 
on  account  of  its  situation,  for  the  attraction  of  the 
sun  on  the  planet  is  capable  of  doing  work.  The  further 
the  planet  is  from  the  sun  the  larger  is  the  quantity  of 
energy  that  it  possesses  from  this  cause.  On  the  other 
hand,  the  further  the  planet  is  from  the  sun  the  smaller 
is  its  velocity,  and  the  less  is  the  quantity  of  energy  that 
it  possesses  of  the  first  kind.  We  unite  the  two  parts, 
and  we  find  that  the  net  result  may  be  expressed  in  the 
following  manner  :  If  a  planet  be  revolving  in  a  circular 
path  round  the  sun,  then  the  total  energy  of  that 
system  (apart  from  any  rotation  of  the  sun  and  planet 
on  their  axes),  when  added  to  the  reciprocal  of  the 
distance  between  the  two  bodies,  measured  with  a  proper 
unit  of  length,  is  the  same  for  all  distances  of  the  same 
two  bodies.  This  shows  the  connection  between  the 
energy  and  the  distance  of  the  planet  from  the  sun. 

Thus  we  see  that  if  the  circle  is  enlarged  the  energy 
of  the  system  increases.  The  moment  of  momentum 
of  the  system  is  proportional  to  the  square  root 
of  the  distance  of  the  two  bodies.  If,  therefore,  the 


236  THE   EARTH'S   BEGINNING. 

distance  of  the  two  bodies  is  increased,  the  moment  of 
momentum  increases  also. 

It  will  illustrate  the  application  of  the  argument 
to  take  a  particular  case  in  which  a  system  of  particles 
is  revolving  round  a  central  sun  in  circular  orbits, 
all  of  which  lie  in  the  same  plane.  Let  us  suppose  that, 
while  the  moment  of  momentum  of  the  system  of  particles 
is  to  remain  unaltered,  one  of  the  particles  is  to  be  shifted 
into  a  plane  which  is  inclined  at  an  angle  of  60°  to  the 
plane  of  the  other  orbits ;  it  can  easily  be  seen  that  an 
area  in  the  new  plane,  when  projected  down  into  the 
original  plane,  will  be  reduced  to  half  its  amount. 
Hence,  as  the  moment  of  momentum  of  the  whole 
system  is  to  be  kept  up,  it  will  be  necessary  for  the 
particle  to  have  a  moment  of  momentum  in  the  circle 
which  it  describes  in  the  new  plane  which  is  double  that 
which  it  had  in  the  original  plane.  It  follows  that  the 
radius  of  the  circle  in  the  new  plane  must  be  four  times 
the  radius  of  the  circle  which  defined  the  orbit  of  the 
particle  in  the  old  plane.  The  energy  of  the  particle  in 
this  orbit  is  therefore  correspondingly  greater,  and  thus 
the  energy  of  the  whole  system  is  increased.  This 
illustrates  how  a  system,  in  which  the  circular  orbits  are 
in  different  planes,  requires  more  energy  for  a  given 
moment  of  momentum  than  would  suffice  if  the  circular 
orbits  had  all  been  in  the  same  plane.  So  long  as  the 
orbits  are  in  different  planes  there  will  still  remain 
a  reserve  of  energy  for  possible  dissipation.  But  the 
dissipation  is  always  in  progress,  and  hence  there  is  an 
incessant  tendency  towards  a  flattening  of  the  system 
by  the  mutual  actions  of  its  parts. 

It  may  help  to  elucidate  this  subject  to  state  the 
matter  as  follows  :  The  more  the  system  contracts, 


THE    CASE    OF  JUPITER.  237 

the  faster  it  must  generally  revolve  ;  this  is  the  universal 
law  when  disturbing  influences  are  excluded.  Take, 
for  instance,  the  sun,  which  is  at  this  moment  con- 
tracting on  account  of  its  loss  of  heat.  In  consequence 
of  that  contraction  it  is  essential  that  the  sun  shall 
gradually  turn  faster  round  on  its  axis.  At  present 
the  sun  requires  twenty-five  days,  four  hours  and 
twenty-nine  minutes  for  each  rotation.  That  period 
must  certainly  be  diminishing,  although  no  doubt  the 
rate  of  diminution  is  very  slow.  Indeed,  it  is  too  slow 
for  us  to  observe ;  nevertheless,  some  diminution  must  be 
in  progress.  Applying  the  same  principle  to  the  primitive 
nebula,  we  see,  that  as  the  contraction  of  the  original 
volume  proceeds,  the  speed  with  which  the  several  parts 
will  rotate  must  increase. 

The  periodic  times  of  the  planets  are  here  instruc- 
tive. The  materials  now  forming  Jupiter  were  situated 
towards  the  exterior  of  the  nebula,  so  that,  as  the 
nebula  contracted,  it  tended  to  leave  Jupiter  behind. 
The  period  in  which  Jupiter  now  revolves  round 
the  sun  may  give  some  notion  of  the  period  of  the 
rotation  of  the  nebula  at  the  time  that  it  extended 
so  far  as  Jupiter.  Subsequently  to  the  formation,  and 
the  detachment  of  Jupiter,  a  body  which  was  hence- 
forth no  longer  in  contact  with  the  nebula,  the  latter 
proceeded  further  in  its  contraction.  Passing  over  the 
intermediate  stages,  we  find  the  nebula  contracting  until 
it  extended  no  further  than  the  line  now  marked  by 
the  earth's  orbit ;  the  speed  with  which  the  nebula  was 
rotating  must  have  been  increasing  all  the  time,  so  that 
though  the  nebula  required  several  years  to  go  round  when 
it  extended  as  far  as  Jupiter,  only  a  fraction  of  that 
period  was  necessary  when  it  had  reached  the  position 


238  THE    EARTHS    BEGINNING. 

indicated  by  the  earth's  track  at  the  present  time. 
Leaving  the  earth  behind  it,  just  as  it  had  previously 
left  Jupiter,  the  nebula  started  on  a  still  further  con- 
densation. It  drew  in,  until  at  last  it  reached  a  further 
stage  by  contraction  into  the  sun,  which  rotates  in 
less  than  a  month.  Thus  the  period  of  Jupiter, 
namely,  twelve  years,  the  period  of  the  earth,  namely, 
one  year,  and  the  period  of  the  sun,  namely,  twenty- 
five  days,  illustrate  the  successive  accelerations  of  the 
rotation  of  the  nebula  in  the  process  of  contraction. 
No  doubt  these  statements  must  be  received  with 
much  qualification,  but  they  will  illustrate  the  nature 
of  the  argument. 

We  may  also  here  mention  the  satellites  of  Uranus, 
all  the  more  so  because  it  has  been  frequently  urged 
as  an  objection  to  the  nebular  theory  that  the  orbits 
of  the  satellites  of  Uranus  lie  in  a  plane  which  is  in- 
clined at  a  very  large  angle;  no  less  than  82°  to  the 
general  plane  of  the  solar  system.  I  shall  refer  in  a 
later  chapter  to  this  subject,  and  consider  what  ex- 
planation can  be  offered  with  regard  to  the  great 
inclination  of  this  plane,  which  is  one  of  the  anomalies 
of  our  system.  For  the  present  I  merely  draw  attention 
to  the  fact  that  the  movements  of  all  four  satellites  of 
Uranus  do  actually  lie  in  the  same  plane,  though,  as 
already  indicated,  it  stands  nearly  at  right  angles  to 
the  ecliptic. 

Professor  Newcomb  has  shown  that  the  four  satel- 
lites of  Uranus  revolve  in  orbits  which  are  almost 
exactly  circular,  and  which,  so  far  as  observation  shows, 
are  absolutely  in  the  same  plane.  From  our  present 
point  of  view  this  is  a  matter  of  much  interest.  What- 
ever may  have  been  the  influence  by  which  this  plane 


THE    ROTATION    OF    URANUS.  239 

departs  so  widely  from  the  plane  of  the  ecliptic,  it 
seems  certain  that  it  must  be  regarded  as  having  acted 
at  a  very  early  period  in  the  evolution  of  the  Uranian 
system ;  and  when  this  system  had  once  started  on  its 
course  of  evolution,  the  operation  of  that  dynamical 
principle  to  which  we  have  so  often  referred  was 
gradually  brought  to  bear  on  the  orbits  of  the  satellites. 
We  have  here  another  isolated  case  resembling  that 
of  Saturn  and  its  rings.  The  fundamental  law  ordained 
that  the  moment  of  momentum  of  Uranus  and  its 
moons  must  remain  constant,  though  the  total  quantity 
of  energy  in  that  system  should  decline.  In  the  course 
of  ages  this  has  led  to  the  adjustment  of  the  orbits 
of  the  four  satellites  into  the  same  plane. 

I  ought  here  to  mention  that  the  rotation  of  Uranus 
on  its  axis  presents  a  problem  which  has  not  yet  been 
solved  by  telescopic  observation.  It  is  extremely  in- 
teresting to  note  that,  as  a  rule,  the  axes  on  which 
the  important  planets  rotate  are  inclined  at  no  great 
angles  to  the  principal  plane  of  the  solar  system.  The 
great  distance  of  Uranus  has,  however,  prevented  astro- 
nomers from  studying  the  rotation  of  that  planet  in  the 
ordinary  manner,  by  observation  of  the  displacement  of 
marks  on  its  surface.  So  far  as  telescopic  observations 
are  concerned,  we  are  therefore  in  ignorance  as  to  the 
axis  about  which  Uranus  revolves.  If,  following  the 
analogy  of  Jupiter,  or  Saturn,  or  Mars,  or  the  earth,  the 
rotation  of  Uranus  was  conducted  about  an  axis,  not 
greatly  inclined  from  the  perpendicular  to  the  ecliptic,  then 
the  rotation  of  Uranus  would  be  about  an  axis  very  far 
from  perpendicular  to  the  plane  in  which  its  satellites 
revolve.  The  analogy  of  the  other  planets  seems  to 
suggest  that  the  rotation  of  a  planet  should  be  nearly 


240  THE    EARTH'S   BEGINNING. 

perpendicular  to  the  plane  in  which  its  satellites  revolve. 
As  the  question  is  one  which  does  not  admit  of  being 
decided  by  observation,  we  may  venture  to  remark 
that  the  necessity  for  a  declining  ratio  of  energy  to 
moment  of  momentum  in  the  Uranian  system  provides  a 
suggestion.  The  moment  of  momentum  of  a  system,  such 
as  that  of  Uranus  and  its  satellites,  is  derived  partly 
from  the  movements  of  the  satellites  and  partly  from  the 
rotation  of  the  planet  itself.  From  the  illustrations  we 
have  already  given,  it  is  plain  that  the  requisite  moment 
of  momentum  is  compatible  with  a  comparatively  small 
energy  only  when  the  system  is  so  adjusted  that 
the  axis  of  rotation  of  the  planet  is  perpendicular  to  the 
plane  in  which  the  satellites  revolve,  or  in  other  words 
when  the  satellites  revolve  in  the  plane  of  the  equator 
of  the  planet.  We  do  not  expect  that  this  condi- 
tion will  be  complied  with  to  the  fullest  extent  in  any 
members  of  the  solar  system.  There  is  indeed  an 
obvious  exception ;  for  the  moon,  in  its  revolution  about 
the  earth,  does  not  revolve  exactly  in  the  earth's 
equator.  We  might,  however,  expect  that  the  tendency 
would  be  for  the  movements  to  adjust  themselves  in 
this  manner.  It  seems  therefore  likely  that  the  direction 
of  the  axis  of  Uranus  is  perpendicular,  or  nearly  so,  to 
the  plane  of  the  movements  of  its  satellites. 

At  this  point  we  take  occasion  to  answer  an  objection 
which  may  perhaps  be  urged  against  the  doctrine  of 
moment  of  momentum  as  here  applied.  I  have  shown 
that  the  tendency  of  this  dynamical  principle  is  to 
reduce  the  movements  towards  one  plane.  It  may  be 
objected  that  if  there  is  this  tendency,  why  is  it  that  the 
movements  have  not  all  been  brought  into  the  same 
plane  exactly  ?  This  has  been  accomplished  in  the  case 


THE    "FLATTENING"    PROCESS.  241 

of  the  bodies  forming  Saturn's  ring,  and  perhaps  in  the 
satellites  of  Uranus.  But  why  is  it  that  all  the  great 
planets  of  our  solar  system  have  not  been  brought  to 
revolve  absolutely  in  the  same  plane  ? 

We  answer  that  the  operations  of  the  forces  by 
which  this  adjustment  is  effected  are  necessarily  ex- 
tremely slow.  The  process  is  still  going  on,  and  it  may 
ultimately  reach  completion.  But  it  is  to  be  particularly 
observed  that  the  nearer  the  approach  is  made  to  the 
final  adjustment,  the  slower  must  be  the  process  of 
adjustment,  and  the  less  efficient  are  the  forces  tending 
to  bring  it  about.  For  the  purpose  of  illustrating  this, 
we  may  estimate  the  efficiency  of  the  forces  in  flattening 
down  the  system  in  the  following  manner.  Suppose 
that  there  are  two  circular  orbits  at  right  angles  to  each 
other,  and  that  we  measure  the  efficiency  of  the  action 
tending  to  bring  the  planes  to  coincide  by  100.  When 
the  planes  are  at  an  angle  of  thirty  degrees  the 
efficiency  is  represented  by  50,  and  when  the  inclination 
is  only  five  degrees  the  efficiency  is  no  more  than  9, 
and  the  efficiency  gradually  lessens  as  the  angle 
declines.  As  the  angles  of  inclination  of  the  planes 
in  the  solar  system  are  so  small,  we  see  that  the 
efficiency  of  the  flattening  operation  in  the  solar  system 
must  have  dwindled  correspondingly.  Hence  we  need 
not  be  surprised  that  the  final  reduction  of  the  orbits 
into  the  same  plane  has  not  yet  been  absolutely 
completed. 

Certainly  the  most  numerous,  and  perhaps  the 
grandest,  illustrations  of  the  operation  of  the  great 
natural  principles  we  have  been  considering  are  to  be 
found  in  the  case  of  the  spiral  nebulse.  The  charac- 
teristic appearance  of  these  objects  demands  special 


242  THE   EARTH'S    BEGINNING. 

explanation,  and  it  is  to  dynamics  we  must  look  for  that 
explanation. 

As  to  the  original  cause  of  a  nebula  we  shall  have 
something  to  say  in  a  future  chapter.  At  present  we 
are  only  considering  how,  when  a  nebula  has  come  into 
existence,  the  action  of  known  dynamical  principles 
will  mould  that  nebula  into  form.  As  an  illustration  of 
a  nebula,  in  what  we  may  describe  as  its  comparatively 
primitive  shape,  we  may  take  the  Great  Nebula  in  Orion. 
This  stupendous  mass  of  vaguely  diffused  vapour  may 
probably  be  regarded  as  in  an  early  stage  when  con- 
trasted with  the  spirals.  We  have  already  shown  how 
the  spectroscopic  evidence  demonstrates  that  the  famous 
nebula  is  actually  a  gaseous  object.  It  stands  thus  in 
marked  contrast  with  many  other  nebulae  which,  by  not 
yielding  a  gaseous  spectrum,  seem  to  inform  us  that 
they  are  objects  which  have  advanced  to  a  further  stage 
in  their  development  than  such  masses  of  mere  glowing 
gas  as  are  found  in  the  splendid  object  in  Orion. 

The  development  of  a  nebula  must  from  dynamical 
principles  proceed  along  the  lines  that  we  have  already 
indicated.  We  shall  assume  that  the  nebula  is  suf- 
ficiently isolated  from  surrounding  objects  in  space  as 
to  be  practically  free  from  disturbing  influences  pro- 
duced by  these  objects.  We  shall  therefore  suppose 
that  the  evolution  of  the  nebula  proceeds  solely  in  con- 
sequence of  the  mutual  attractions  of  its  various  parts. 
In  its  original  formation  the  nebula  receives  a  certain 
endowment  of  energy  and  a  certain  endowment  of 
moment  of  momentum;  the  mere  fact  that  we  see 
the  nebula,  the  fact  that  it  radiates  light,  shows  that 
it  must  be  expending  energy,  and  the  decline  of  the 
energy  will  proceed  continuously  from  the  formation 


A    NEBULA    TENDS    TO   A    FLAT   FORM.        243 

of  the  object.  The  laws  of  dynamics  assure  us  that 
no  matter  what  may  be  the  losses  of  energy  which 
the  nebula  suffers  through  radiation  or  through  the 
collisions  of  its  particles,  or  through  their  tidal  actions, 
or  in  any  way  whatever  from  their  mutual  actions,  the 
moment  of  momentum  must  remain  unchanged. 

As  the  ages  roll  by,  the  nebula  must  gradually  come 
to  dispose  itself,  so  that  the  moment  of  momentum  shall 
be  maintained,  notwithstanding  that  the  energy  may 
have  wasted  away  to  no  more  than  a  fraction  of  its 
original  amount.  Originally  there  was,  of  course,  one 
plane,  in  which  the  moment  of  momentum  was  a 
maximum.  It  is  what  we  have  called  the  principal 
plane  of  the  system,  and  the  evolution  tends  in  the 
direction  of  making  the  nebula  gradually  settle  down 
towards  this  plane.  We  have  seen  that  the  moment 
of  momentum  can  be  sustained  with  the  utmost  economy 
of  energy  by  adjusting  the  movements  of  the  particles 
so  that  they  all  take  place  in  orbits  parallel  to  this 
plane,  and  the  mutual  attractions  of  the  several  parts 
will  gradually  tend  to  bring  the  planes  of  the  different 
orbits  into  coincidence.  Every  collision  between  two  atoms, 
every  ray  of  light  sent  forth,  conduce  to  the  final  result. 
Hence  it  is  that  the  nebula  gradually  tends  to  the  form 
of  a  flat  plane.  This  is  the  first  point  to  be  noticed  in 
the  formation  of  a  spiral  nebula. 

But  there  is  a  further  consideration.  As  the  nebula 
radiates  its  light  and  its  heat,  and  thus  loses  its  energy, 
it  must  be  undergoing  continual  contraction.  Con- 
currently with  its  gradual  assumption  of  a  flat  form, 
the  nebula  is  also  becoming  smaller.  Here  again  that 
fundamental  conception  of  the  conservation  of  moment 
of  momentum  will  give  us  important  information.  If 


244  THE    EARTH'S    BEGINNING. 

the  nebula  contracts,  that  is  to  say,  if  each  of  its  par- 
ticles draws  in  closer  to  the  centre,  the  orbits  of  each 
of  its  particles  will  be  reduced.  But  the  quantity  of 
areas  to  be  described  each  second  must  be  kept  up. 
We  have  pointed  out  that  it  is  infinitely  improbable 
the  system  should  have  been  started  without  any 
moment  of  momentum,  and  this  condition  of  affairs 
being  infinitely  improbable,  we  dismiss  any  thought  of 
its  occurrence.  As  the  particles  settle  towards  the  plane, 
the  areas  swept  out  by  the  movements  to  the  right,  and 
those  areas  swept  out  by  the  movements  to  the  left,  will 
not  be  identical ;  there  will  therefore  be  a  balance  on 
one  side,  and  that  balance  must  be  maintained  without 
the  slightest  alteration  throughout  all  time.  As  the 
particles  get  closer  together,  and  as  their  orbits  lessen, 
it  will  necessarily  happen  that  the  velocities  of  the 
particles  must  increase,  for  not  otherwise  can  the 
fundamental  principle  of  the  constant  moment  of 
momentum  be  maintained.  And  as  the  system  gets 
smaller  and  smaller,  by  contraction  from  an  original 
widely  diffused  nebulosity,  like,  perhaps,  the  nebula  in 
Orion,  down  to  a  spiral  nebula  which  may  occupy  not  a 
thousandth  or  a  millionth  part  of  the  original  volume, 
the  areas  will  be  kept  up  by  currents  of  particles  moving 
in  the  two  opposite  ways  around  a  central  point.  As 
the  contraction  proceeds,  the  opposing  particles  will 
occasionally  collide,  and  consequently  the  tendency  will 
be  for  the  predominant  side  to  assert  itself  more  and 
'more,  until  at  last  we  may  expect  a  condition  to  be 
reached  in  which  all  the  movements  will  take  place  in 
one  direction,  and  when  the  sum  of  the  areas  described 
in  a  second,  by  each  of  the  particles,  multiplied  by  their 
respective  masses,  will  represent  the  original  endowment 


WHY    THEY  ARE    SPIRAL.  245 

of  moment  of  momentum.  Thus  we  find  that  the 
whole  object  becomes  ultimately  possessed  of  a  move- 
ment of  rotation. 

The  same  argument  will  show  that  the  inner  parts  of 
the  nebula  will  revolve  more  rapidly  than  those  in  the 
exterior.  Thus  we  find  the  whirlpool  structure  pro- 
duced, and  thus  we  obtain  an  explanation,  not  only  of 
the  flatness  of  the  nebula,  but  also  of  the  spiral  form 
which  it  possesses.  It  is  not  too  much  to  say  that  the 
operation  of  the  causes  we  have  specified,  if  external 
influence  be  withheld,  tends  ultimately  to  produce  the 
spiral,  whatever  may  have  been  the  original  form  of  the 
object.  No  longer,  therefore,  need  we  feel  any  hesitation 
in  believing  the  assurance  of  Professor  Keeler  that  out 
of  the  one  hundred  and  twenty  thousand  nebulae,  at  least 
one-half  must  be  spirals.  We  have  found  in  dynamics 
an  explanation  of  that  remarkable  type  of  object  which 
we  have  now  reason  to  think  is  one  of  the  great 
fundamental  forms  of  nature. 


17 


CHAPTER    XII. 

THE   EVOLUTION  OF  THE  SOLAR   SYSTEM. 

The  Primaeval  Nebula — A  Planetary  Nebula — The  Progress  of  its 
Evolution — Unsymmetrical  Contraction— Centres  of  Condensation — 
The  Form  ultimately  assumed — Difference  between  Small  Bodies 
and  Large — Earth  and  Sun — Acceleration  of  Velocities — Formation 
of  the  Subordinate  Systems — Special  Circumstances  in  the  case  of 
the  Earth  and  Moon — Vast  Scale  of  the  Spirals — Spectra  of 
the  Spiral  Nebulae. 

WE  shall  consider  in  this  chapter  what  we  believe  to 
have  been  the  history  of  that  splendid  system,  formed 
by  the  planets  under  the  presiding  control  of  the  sun. 
The  ground  over  which  we  have  already  passed  will 
prepare  us  for  the  famous  doctrine  that  the  sun,  the 
planets  and  their  satellites,  together  with  the  other 
bodies  which  form  the  group  we  call  the  solar  system, 
have  originated  from  the  contraction  of  a  primaeval 
nebula. 

As  the  ages  rolled  by,  this  great  primseval  nebula 
began  to  undergo  modification.  In  accordance  with  the 
universal  law  which  we  find  obeyed  in  our  laboratories, 
and  which  we  have  reason  to  believe  must  be  equally 
obeyed  throughout  the  whole  extent  of  space,  this  nebula, 
if  warmer  than  the  surrounding  space,  must  begin  to 


THE    BEGINNING    OF   THE   SOLAR    SYSTEM.      247 

radiate  forth  its  heat.  We  are  to  assume  that  the 
nebula  does  not  receive  heat  from  other  bodies,  adequate 
to  compensate  for  that  which  it  dissipates  by  radiation. 
There  is  thus  a  loss  of  heat  and  consequently  the  nebula 
must  begin  to  contract.  Its  material  must  gradually 
draw  together,  and  must  do  so  under  the  operation  of 
those  fundamental  laws  which  we  have  explained  in  the 
last  chapter. 

The  contraction,  or  rather  the  condensation,  of  the 
material  would  of  course  generally  be  greatest  at  the 
central  portion  of  the  nebula.  This  is  especially  notice- 
able in  the  photograph  of  the  great  spiral  already  re- 
ferred to.  But  in  addition  to  this  special  condensation  at 
the  centre,  the  concentration  takes  place  also,  though  in 
a  lesser  degree,  at  many  other  points  throughout  the 
whole  extent  of  the  glowing  mass.  Each  centre  of  con- 
densation which  in  this  way  becomes  established  tends 
continually  to  increase.  In  consequence  of  this  law,  as 
the  great  nebula  contracted  and  as  the  great  bulk  of  the 
material  drew  in  towards  the  centre,  there  were  isolated 
regions  in  the  nebula  which  became  subordinate  centres 
of  condensation.  Perhaps  in  the  primseval  nebula,  from 
which  the  solar  system  originated,  there  were  half-a- 
dozen  or  more  of  these  centres  that  were  of  conspicuous 
importance,  while  a  much  larger  number  of  small  points 
were  also  distinguished  from  the  surrounding  nebula. 
(Figs.  40  and  41.)  And  still  the  contraction  went  on.  The 
heat,  or  rather  the  energy  with  which  the  nebula  had  been 
originally  charged,  was  still  being  dissipated  by  radiation. 
We  give  no  estimate  of  the  myriads  of  years  that 
each  stage  of  the  mighty  process  must  have  occupied. 
The  tendency  of  the  transformation  was,  however,  always 
in  one  direction.  It  did  at  last  result  in  a  great  increase 


248  THE    EARTH'S    BEGINNING. 

of  the  density  of  the  substance  of  the  nebula,  both  in  the 
central  regions  as  well  as  in  the  subordinate  parts.  In 
due  time  this  increase  in  density  had  reached  such  a 
point  that  the  materials  in  the  condensing  centres  could 
be  no  longer  described  as  retaining  the  gaseous  form. 

But  though  heat  was  incessantly  being  radiated  from 
the  great  nebula,  it  did  not  necessarily  follow  that  the 
nebula  was  itself  losing  temperature.  This  is  a  seeming 
paradox  to  which  we  have  already  had  occasion  to  refer 
in  Chapter  VI.  We  need  not  now  further  refer  to  it 
than  to  remember  that,  in  speaking  of  the  loss  of  heat 
from  the  nebula,  it  would  sometimes  not  be  correct  to 
describe  the  operation  as  that  of  cooling.  Up  to  a  certain 
stage  in  the  condensation,  the  loss  of  heat  leads  rather  to 
an  augmentation  of  temperature  than  to  its  decline. 

We  are  thus  led  to  see  how  the  laws  of  heat,  after 
being  in  action  on  the  primitive  nebula  for  a  period 
of  illimitable  ages,  have  at  last  effected  a  marvellous 
transformation.  That  nebula  has  condensed  into  a  vast 
central  mass  with  a  number  of  associated  subordinate 
portions.  We  may  suppose  that  the  original  nebula  in 
the  course  of  time  does  practically  disappear.  It  is 
absorbed  by  the  attraction  of  those  ponderous  centres 
which  have  gradually  developed  throughout  its  extent. 

The  large  central  body,  and  perhaps  some  of  the  other 
bodies  thus  evolved,  are  at  first  of  so  high  a  temperature 
that  a  copious  radiation  of  heat  still  goes  forth  from  the 
system.  As  they  discharge  their  stores  of  heat,  the 
smaller  bodies  show  the  effects  of  loss  of  heat  more 
rapidly  than  those  which  are  larger.  It  is  indeed 
obvious  that  a  small  body  must  cool  more  rapidly  than 
a  big  one.  It  is  sufficient  to  note  that  the  cooling  takes 
place  from  the  surface,  and  that  the  bigger  the  body  the 


TWO    BODIES    AS    EXAMPLES.  249 


FIG.  38. — THE  RING  NEBULA  IN  LYRA   (Lick  Observatory). 
(From  the  Royal  Astronomical  Society  Series.) 

larger  the  quantity  ot  material  that  it  contains  for  each 
unit  ot  superficial  area.  If  the  radius  of  a  sphere  be 
doubled,  its  volume  is  increased  eightfold,  while  its 
surface  is  only  increased  fourfold. 

Let  us  now  concentrate  our  attention  on  two  of  the 
bodies  which,  after  immense  ages,  have  been  formed 
from  the  condensation  of  the  primaeval  nebula.  Let  one 
of  the  two  bodies  be  that  central  object,  which  prepon- 
derates so  enormously  that  its  mass  is  a  thousandfold 
that  of  all  the  others  taken  together.  Let  the  other  be 
one  of  the  smaller  bodies.  As  it  parts  with  its  heat, 
the  smaller  body,  which  has  originally  condensed  from 
the  nebula,  will  assume  some  of  the  features  of  a  mass 
of  molten  liquid.  From  the  liquid  condition,  the  body 
will  pass  with  comparative  rapidity  into  a  solid  state,  at 
least  on  its  outer  parts.  The  exterior  of  this  body  will 
therefore  become  solid  while  the  interior  is  still  at  an 
excessively  high  temperature.  The  outer  material, 


250  THE   EARTH'S    BEGINNING. 

which  has  assumed  the  solid  form,  is  constituted  of  the 
elements  with  which  we  are  acquainted,  and  is  in 
the  form  of  what  the  geologist  would  class  as  the- 
igneous  rocks,  of  which  granite  is  the  best  known 
example.  The  shell  of  hard  rocks  outside  encloses 
the  material  which  is  still  heated  and  molten  inside. 
Such  a  crust  would  certainly  be  an  extremely  bad 
conductor  of  heat.  The  internal  heat  is  therefore 
greatly  obstructed  in  its  passage  outwards  to  the 
surface.  The  internal  heat  may  consequently  be  pre- 
served in  the  interior  of  the  body  for  an  enormously 
protracted  period,  a  period  perhaps  comparable  with 
those  immense  ages  which  the  evolution  of  the  body 
from  the  primaeval  nebula  has  demanded.  The 
smaller  body  may  have  thus  attained  a  condition  in 
which  the  temperature  reigning  on  its  surface  is  regu- 
lated chiefly  by  the  external  conditions  of  the  space 
around,  while  the  internal  parts  are  still  highly  charged 
with  the  primitive  heat  from  the  original  nebula. 

The  great  central  mass,  which  we  may  regard  as 
thousands  of  times  greater  than  that  of  the  subordinate 
body,  cools  much  more  slowly.  The  cooling  of  this 
great  mass  is  so  enormously  protracted  in  comparison 
with  that  of  the  smaller  body  that  it  is  quite  conceivable 
the  central  mass  may  continue  to  glow  with  intense 
fervour  for  immense  ages  after  the  smaller  body  has 
become  covered  with  hard  rock. 

It  will,  I  hope,  be  clear  that  the  two  bodies  to  which 
I  am  here  alluding  are  not  merely  imaginary  objects. 
The  small  body,  which  has  so  far  cooled  down  that 
its  surface  has  lost  all  indication  of  internal  heat,  is 
of  course  our  earth.  The  great  central  mass  which 
still  glows  with  intense  fervour  is  the  sun.  Such  is. 


MODIFYING    CIRCUMSTANCES.  251 

in  outline  the  origin  of  the  sun  and  the  earth  as  sug- 
gested by  the  nebular  theory. 

What  we  have  said  of  the  formation  of  the  earth 
will  equally  apply  to  the  evolution  of  other  detached 
portions  of  the  primitive  nebula.  There  may  be  several 
of  these,  and  they  may  vary  greatly  in  size.  The 
smaller  they  are  the  more  rapidly  in  general  will 
the  superabundant  heat  be  radiated  away,  and  the 
sooner  will  the  surface  of  that  planet  acquire  the 
temperature  which  is  determined  by  the  surrounding 
conditions.  There  are,  however,  many  modifying  cir- 
cumstances. 

It  is  essential  to  notice  that  the  primaeval  nebula 
must  have  had  some  initial  moment  of  momentum, 
unless  we  are  to  assume  the  occurrence  of  that  which 
is  infinitely  improbable.  It  would  have  been  infinitely 
improbable  for  the  system  not  to  have  had  some 
moment  of  momentum  originally.  As  the  evolution 
proceeds,  and  as  the  energy  is  expended,  while  this 
original  endowment  of  moment  of  momentum  is  pre- 
served, we  find,  as  explained  in  the  last  chapter,  the 
system  gradually  settling  down  into  proximity  to  a 
plane,  and  gradually  acquiring  a  uniform  direction  of 
revolution.  Hence  we  see  that  each  of  the  subordinate 
masses  which  ultimately  consolidate  to  form  a  planet 
have  a  motion  of  revolution  around  the  central 
body.  In  like  manner  the  central  body  itself  rotates, 
and  all  these  motions  are  performed  in  the  same 
direction. 

In  addition  to  the  revolutions  of  the  planets  around 
the  sun,  there  are  other  motions  which  can  be  accounted 
for  as  consequences  of  the  contraction  of  the  nebula. 
We  now  refer  to  that  central  portion  which  is  to  form 


252  THE   EARTH'S   BEGINNING. 

the  sun,  and  consider,  in  the  first  instance,  only  one 
of  the  subordinate  portions  which  is  to  form  a  planet. 
As  these  two  bodies  form  part  of  the  same  nebulous 
mass  they  will  to  a  certain  extent  rotate  together  as  one 
piece.  If  any  body  is  rotating  as  a  whole,  every  part  of 
that  body  is  also  in  actual  rotation.  We  shall  refer  to 
this  again  later  on  ;  but  for  the  present  it  is  sufficient  to 
observe  that  as  the  planet  was  originally  continuous 
with  the  sun,  it  had  a  motion  of  rotation  besides  its 
motion  of  revolution,  and  it  revolved  round  its  own 
axis  in  a  period  equal  to  that  of  its  revolution  round 
the  sun.  In  the  beginning  the  rotation  of  the  planet 
was  therefore  an  exceedingly  slow  movement.  But  it 
became  subsequently  accelerated.  For  we  have  already 
explained  that  each  planet  is  by  itself  subjected  to  the 
law  of  the  conservation  of  moment  of  momentum.  As 
each  planet  assumes  a  separate  existence,  it  draws  to 
itself  its  share  of  the  moment  of  momentum,  and  that 
must  be  strictly  preserved.  But  the  planet,  or  rather 
the  materials  which  are  to  form  the  future  planet,  are 
all  the  time  shrinking ;  they  are  drawing  more  closely 
together.  If,  therefore,  the  area  which  each  particle  of 
the  planet  describes  when  multiplied  by  the  mass  of  that 
particle  and  added  to  the  similar  products  arising  from 
all  the  other  particles,  is  to  remain  constant,  it  becomes 
necessary  that  just  as  the  orbits  of  these  particles 
diminish  in  size,  so  must  the  speed  at  which  they 
revolve  increase.  We  thus  find  that  there  is  a  tendency 
in  the  planet  to  accelerate  its  rotation.  And  thus  we 
see  that  a  time  will  come  when  the  planet,  having 
assumed  an  independent  existence,  will  be  found 
rotating  round  its  axis  with  a  velocity  which  must 
be  considered  high  in  comparison  with  the  angular 


THE  EARTH  AND  THE  MOON.       253 

velocity  which  the  planet  had  while  it  still  formed  part 
of  the  original  nebula. 

As  the  planets  have  been  evolved  so  as  to  describe 
their  several  orbits  around  the  sun,  so  in  like  manner 
the  smaller  systems  of  satellites  have  been  so  evolved  as 
to  describe  their  orbits  round  the  several  planets  that 
are  their  respective  primaries.  When  a  planet,  or  rather 
the  materials  which  were  drawing  together  to  form  a 
planet,  had  acquired  a  predominant  attraction  for  the 
parts  of  the  primseval  nebula  in  their  locality,  a  portion 
of  the  nebulous  material  became  specially  associated 
with  the  planet.  As  the  planet  with  this  nebulous 
material  became  separated  from  the  central  contracting 
sun,  or  became,  as  it  were,  left  behind  while  the  sun  was 
drawing  into  itself  the  material  which  surrounded  it, 
the  planet  and  its  associated  nebula  underwent  on  a 
miniature  scale  an  evolution  similar  to  that  which  had 
already  taken  place  in  the  formation  of  the  sun  and  the 
planets  as  a  whole.  In  this  manner  secondary  systems 
seem  sometimes  to  have  had  their  origin. 

We  should,  however,  say  that  though  what  we  have 
here  indicated  appears  to  explain  fully  the  evolution  of 
some  of  the  systems,  such,  for  instance,  as  that  of  Jupiter 
and  his  four  moons,  or  Saturn  and  his  eight  or  nine,  the 
circumstances  with  regard  to  the  earth  and  the  moon 
are  such  as  to  require  a  very  different  explanation  of  the 
origin  of  our  satellite.  In  the  first  place  we  may  notice 
that  the  great  mass  of  the  moon,  in  comparison  with  the 
earth,  is  a  wholly  exceptional  feature  in  the  relations 
between  the  planets  and  their  satellites  in  the  other 
parts  of  the  system.  In  no  other  instance  does  the  mass 
of  a  satellite  bear  to  the  mass  of  the  planet  a  ratio  any- 
thing like  so  great  as  the  ratio  of  our  moon  to  the  earth. 


254  THE   EARTH'S   BEGINNING. 

The  moon  has  a  mass  which  is  about  one-eightieth  of  the 
mass  of  the  earth,  while  even  the  largest  of  Jupiter's 
satellites  has  not  one  ten-thousandth  part  of  the  mass  of 
the  planet  itself.  The  evolution  of  the  earth  and  moon 
system  has  been  brought  about  in  a  manner  very 
different  from  that  of  the  evolution  of  the  other  systems 
of  satellites.  We  do  not  here  enter  into  any  discussion 
of  the  matter.  We  merely  remind  the  reader  that  it  is 
now  known,  mainly  by  the  researches  of  Professor 
G.  H.  Darwin,  that  in  all  probability  the  moon  was 
originally  part  of  the  earth,  and  that  a  partition  having 
occurred  while  the  materials  of  the  earth  and  moon 
were  still  in  a  plastic  state,  a  small  portion  broke  away 
to  form  the  moon,  leaving  behind  the  greater  mass  to 
form  the  earth.  Then,  under  the  influence  of  tides, 
which  may  agitate  a  mass  of  molten  rock,  as  the  moon 
was  once  (Fig.  39),  just  as  they  may  agitate  an  ocean, 
the  moon  was  forced  away,  and  was  ultimately  conducted 
to  its  present  orbit. 

It  was  at  first  tempting  to  imagine  that  a  theory 
which  accounted  so  satisfactorily  for  the  evolution  of 
the  moon  from  the  earth  might  also  account  in  a 
similar  manner  for  the  evolution  of  the  earth  from  the 
sun.  Had  this  been  the  case,  it  is  needless  to  say  that 
the  principles  we  now  accept  in  the  nebular  theory 
would  have  needed  large  modification,  if  not  actual 
abandonment.  A  close  examination  into  the  actual 
statistics  brings  forcibly  before  us  the  exceptional 
character  of  the  earth-moon  system.  It  can  be  de- 
monstrated that  the  earth  could  not  have  been  evolved 
from  the  sun  in  the  same  manner  as  there  is  every 
reason  to  believe  that  the  moon  has  been  evolved  from 
the  earth.  The  evolution  of  the  satellites  of  Jupiter 


EVOLUTION   OF    THE   PLANETS.  255 


Fig.  39. — LUNAR  CRATERS:  HYGINUS  AND  ALBATEGNIUS. 

(Photographed  by  MM.  Loewy  and  Puiseux.) 

has  proceeded  along  lines  quite  different  from  those 
of  the  evolution  of  the  moon  from  the  earth,  so  that 
we  may,  perhaps,  find  in  the  evolution  of  the  satellites 
of  Jupiter  an  illustration  in  miniature  of  the  way  in 


256  THE   EARTH'S    BEGINNING. 

which  the  planets  themselves  have  been  evolved  in 
relation  to  the  sun. 

We  must  not  forget  that  the  only  spiral  nebulae 
which  lie  within  the  reach  of  our  powers  of  observation, 
whether  telescopic  or  photographic,  appear  to  be  objects 
of  enormously  greater  cosmical  magnificence  than  was 
that  primaeval  nebula  from  which  so  insignificant  an 
object  as  the  solar  system  has  sprung.  The  great  spirals, 
so  far  as  we  can  tell  at  present,  appear  to  be  thousands 
of  times,  or  even  millions  of  times,  greater  in  area  than 
the  solar  system.  At  this  point,  however,  we  must 
speak  with  special  caution,  having  due  regard  to  the 
paucity  of  our  knowledge  of  a  most  important  element. 
Astronomers  must  confess  that  no  efforts  which  have 
yet  been  made  to  determine  the  dimensions  of  a  nebula 
have  been  crowned  with  success.  We  have  not  any 
precise  idea  as  to  what  the  distance  of  the  great  spiral 
might  be.  We  generally  take  for  granted  that  these 
nebulae  are  at  distances  comparable  with  the  distances 
of  the  stars.  On  this  assumption  we  estimate  that 
the  spiral  nebulae  must  transcend  enormously  the 
dimensions  of  the  primaeval  nebula  from  which  the 
solar  system  has  sprung.  The  spiral  nebulae  that 
have  so  far  come  within  our  observation  seem  to  be 
objects  of  an  order  of  magnitude  altogether  higher 
than  a  solar  system.  They  seem  to  be  engaged  on 
the  majestic  function  of  evolving  systems  of  stars  like 
the  Milky  Way,  rather  than  on  the  inconsiderable 
task  of  producing  a  system  which  concerns  only  a 
single  star  and  not  a  galaxy. 

The  spiral  form  of  structure  is  one  in  which  Nature 
seems  to  delight.  We  find  it  in  the  organic  world 
allied  with  objects  of  the  greatest  interest  and  beauty. 


THE    SPIRAL   FORM   IN  NATURE.  257 


Fig-.  40. — A  REMARKABLE  SPIRAL  (n.g.c.  628 ;  in  Pisces). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

The  ammonite,  a  magnificent  spiral  shell  sometimes 
exceeding  three  feet  in  diameter,  belongs  to  a  type 
which  dominated  the  waters  of  the  globe  in  secondary 
times,  and  which  still  survives  in  the  nautilus.  The 
same  form  is  reproduced  in  minute  creations  totally 
different  from  ammonites  in  their  zoological  relations. 
Among  the  exquisite  foraminifera  which  the  microscopist 
knows  so  well  may  be  found  most  delicate  and  beautiful 
spirals.  Just  as  we  see  every  range  of  spiral  in  the 
animal  world,  from  an  organism  invisible  to  the  naked 
eye,  up  to  an  ammonite  a  yard  or  more  across,  so  it 


258  THE   EARTH'S   BEGINNING. 

would  seem  that  there  are  spiral  nebulae  ranging  from 
such  vast  objects  as  the  great  spiral  in  Canes  Venatici 
down  to  such  relatively  minute  spirals  as  those  whose 
humble  function  it  is  to  develop  a  solar  system.  It 
is  no  more  than  a  reasonable  supposition  that  the 
great  spirals  in  the  heavens  are  probably  only  the 
more  majestic  objects  of  an  extremely  numerous  class. 
The  smaller  objects  of  this  type — among  which  we 
might  expect  to  find  nebulae  like,  in  size  and  import- 
ance, to  the  primaeval  nebula  of  our  system — are  so 
small  that  they  have  not  yet  been  recognised. 

It  should  at  this  stage  be  mentioned  that  several 
curious  small  planetary  nebulae  have  in  these  modern 
days  been  discovered  by  their  peculiar  spectra.  If 
the  nebulous  character  of  these  most  interesting 
objects  had  not  been  accidentally  disclosed  by  char- 
acteristic lines  in  their  spectra,  these  undoubted 
nebulae  would  each  have  been  classified  merely  as 
stars.  This  fact  will  lead  us  to  the  surmise  that 
there  must  be  myriads  of  nebulae  in  the  heavens, 
too  small  to  come  within  the  range  of  our  telescopes 
or  of  our  most  sensitive  photographic  plates.  Suppose 
that  a  facsimile  of  the  primaeval  nebula  of  our  system, 
precisely  corresponding  with  it  in  size  and  identical 
with  it  in  every  detail,  were  at  the  present  moment 
located  in  space,  but  at  a  distance  from  our  stand- 
point, as  great  as  the  distance  of,  let  us  say,  the 
great  spiral;  it  seems  certain  that  this  nebula,  even 
though  it  contained  the  materials  for  a  huge  sun  and 
a  potential  system  of  mighty  planets,  if  not  actually 
invisible  to  us  here,  would  in  all  probability  demand 
the  best  powers  of  our  instruments  to  reveal  it,  and 
then  it  would  be  classed  not  as  a  nebula  at  all  but 


SMALL   PLANETARY  NEBULM 


259 


Fig.  41.— A  CLEARLY  CUT  SPIRAL  (n.g.c.  4321  ;  in  Coma  Berenices). 

(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

as  a  star  of  perhaps  the  12th  or  15th,  or  even  smaller 
magnitude. 

It  is  to  be  remembered  that  the  class  of  minute 
planetary  nebula  make  themselves  known  solely  by  the 
fact  that  they  exhibit  the  bright  line  indicative  of 
gaseous  spectra.  If  these  objects  (though  still  nebulae) 
had  not  displayed  gaseous  spectra,  it  is  certain  they 
would  have  escaped  detection,  at  least  by  the  process 
which  has  actually  proved  so  successful.  The  con- 
tinuous band  of  light  which  they  would  then  have 
presented  could  not  be  discriminated  from  the  band  of 


260  THE   EARTH'S    BEGINNING. 

light  from  a  star.  It  is  therefore  not  improbable  that 
among  the  star-like  bodies  which  have  been  represented 
on  our  photographs,  there  may  be  some  which  are 
really  minute  spiral  nebulas.  In  general  a  star  is  a 
minute  point  of  light  which  no  augmentation  of  tele- 
scopic power  and  no  magnification  will  show  otherwise 
than  as  a  point,  granted  only  good  optical  conditions 
and  good  opportunity  so  far  as  the  atmosphere  is 
concerned.  It  has,  however,  been  occasionally  noted 
that  certain  so-called  stars  are  not  mere  points  of 
light ;  they  do  possess  what  is  described  as  a  disc.  It  is 
not  at  all  impossible  that  the  objects  so  referred  to  are 
spiral  nebulae.  We  may  describe  them  as  formed  on  a 
small  scale  in  comparison  with  the  great  spiral  or  the 
nebula  in  Andromeda.  But  the  smallness  here  referred 
to  is  only  relative.  They  are  in  all  probability  quite  as 
vast  as  the  primaeval  spiral  nebula  from  which  the  solar 
system  has  been  evolved,  though  not  so  large  as  those 
curious  ring-shaped  nebulae  of  which  the  most  cele- 
brated example  lies  in  the  constellation  Lyra  (Fig.  38). 
Such  is  an  outline  of  what  we  believe  to  have  been 
the  history  of  our  solar  system.  We  have  already  given 
the  evidence  derived  from  the  laws  of  heat.  We  have 
now  to  consider  the  evidence  which  has  been  derived 
from  the  constitution  of  the  system  itself.  We  shall  see 
how  strongly  it  supports  the  belief  that  the  origin  of 
sun  and  planets  has  been  such  as  the  nebular  theory 
suggests. 


CHAPTER  XIII. 

THE  UNITY  OF  MATERIAL  IN  THE  HEAVENS  AND 
THE  EARTH. 

Clouds — Fire-Mist — Vapour  of  Platinum — Components  of  Chalk — Con- 
stituents of  the  Primaeval  Fire-Mist — Objections — Origin  of  the 
Mist — Remarkable  Discovery  of  the  Century — Analysis  of  the  Sun — 
Spectroscopic  Analysis — Simplicity  of  Solar  Chemistry — Potassium 
—A  Drop  of  Water— The  Solar  Elements— Calcium— The  Most  Im- 
portant Lines  in  the  Solar  Spectrum — Photograph  of  the  Sun — 
Carbon  in  the  Solar  Clouds — Function  of  Carbon — Bunsen's  Burner 
Illustrates  Carbon  in  the  Sun — Carbon  Vapours  in  the  Sun — The 
Supposed  Limit  to  our  Knowledge  of  the  Heavens — Characteristics 
of  Spectroscopic  Work — Bearing  on  the  Nebular  Theory. 

IN  considering  how  the  formation  of  our  solar  system 
was  brought  about,  we  naturally  first  enquire  as  to  the 
material  of  which  this  superb  scheme  is  constructed. 
What  were  the  materials  already  to  hand  from  which, 
in  pursuance  of  the  laws  of  Nature,  the  solar  system  was 
evolved  ? 

See  the  robust  and  solid  nature  of  this  earth  of 
ours,  and  the  robust  and  solid  nature  of  the  moon  and 
the  planets.  It  might  at  first  sight  be  concluded  that 
the  primitive  materials  of  our  earth  had  also  been  in 
the  solid  state.  But  such  is  not  the  case.  The  primi- 
tive material  of  the  solar  system  was  not  solid,  it  was 
18 


262  TEE   EARTH'S   BEGINNING. 

not  even  liquid.  What  we  may  describe  as  the  mother- 
substance  of  the  universe  must  have  been  of  quite  a 
different  nature ;  we  can  give  an  illustration  of  the 
physical  character  of  that  substance. 

The  lover  of  Nature  delights  to  look  at  the  moun- 
tains and  the  trees,  the  lakes  and  the  rivers.  But  he 
will  not  confine  his  regard  merely  to  the  objects  on 
the  earth's  surface.  He,  no  less  than  the  artist  and  the 
poet,  delights  to  gaze  at  that  enchanting  scenery  which, 
day  by  day,  is  displayed  in  infinite  beauty  overhead; 
that  scenery  which  is  not  wholly  withheld  even  from 
observers  whose  lives  may  be  passed  amid  the  busy 
haunts  of  men,  that  scenery  which  is  so  often  displayed 
on  fine  days  at  all  seasons.  We  are  alluding  to  those 
clouds  which  add  the  charm  of  infinite  variety  to 
the  sky  above  us. 

It  is  necessary  for  us  now  to  think  of  matter 
when  it  possesses  neither  the  density  of  a  solid,  nor 
the  qualities  of  a  liquid,  but  rather  when  it  has  that 
delicate  texture  which  the  clouds  exhibit.  The  primaeval 
material  from  which  the  solar  system  has  been  evolved 
is  of  a  texture  somewhat  similar  to  that  of  the  clouds. 
This  primeeval  material  is  neither  solid  nor  liquid  ;  it 
is  what  we  may  describe  as  vapour. 

But  having  pointed  to  the  clouds  in  our  own  sky 
as  illustrating,  in  a  sense,  the  texture  of  this  original 
mother-substance  of  the  solar  system,  we  can  carry  the 
analogy  no  further.  Those  dark  and  threatening  masses 
which  forbode  the  thunderstorm,  or  those  beautiful  fleecy 
clouds  which  enhance  the  loveliness  of  a  summer's  day, 
are,  of  course,  merely  the  vapours  of  water.  But  the 
vapours  in  the  mother-substance  from  which  systems 
have  been  evolved  were  by  no  means  the  vapours  of 


PLATINUM.  263 

water.  They  were  vapours  of  a  very  different  character 
— vapours  that  suggest  the  abodes  of  Pluto  rather  than 
the  gentle  rain  that  blesses  the  earth.  In  the  mother- 
substance  of  the  solar  system  vapours  of  a  great 
Variety  of  substances  were  blended.  For  in  the  potent 
laboratory  of  Nature  every  substance,  be  it  a  metal  or 
any-  other  element,  or  any  compound,  no  matter  how 
refractory,  will,  under  suitable  circumstances,  be  dis- 
solved into  vapour. 

Take,  for  instance,  such  a  material  as  platinum. 
Could  anything  be  less  like  a  vapour  than  this  silvery 
metal?  We  know  that  platinum  is  the  densest  of  all 
the  elements.  We  know  that  platinum,  more  effectually 
than  other  metals,  resists  liquefaction  from  the  applica- 
tion of  heat.  No  ordinary  furnace  can  fuse  platinum ; 
yet  in  another  way  we  can  overcome  the  resistance  of 
this  metal.  The  electric  arc,  when  suitably  managed, 
yields  a  temperature  higher  than  that  of  any  furnace. 
Let  the  electric  current  spring  from  one  pole  of  platinum 
to  another,  and  a  brilliant  arc  of  light  is  produced  by 
the  glowing  gas,  which  is  characteristic  of  platinum. 
The  light  dispensed  from  that  arc  is  different  from  the 
light  that  would  be  radiated  if  the  poles  were  of  any 
material  other  than  platinum.  Some  of  the  platinum 
has  not  alone  been  melted,  it  has  actually  been  turned 
into  vapour  by  the  overpowering  heat  to  which  it  has 
been  subjected.  Thus  the  solidity  of  this  substance, 
which  resists  so  stubbornly  the  action  of  lower  tem- 
peratures, can  be  overcome,  and  the  very  densest  of 
all  metals  is  dissolved  into  wisps  of  vapour. 

We  choose  the  case  of  platinum  as  an  illustration 
because  it  is  a  substance  exceptionally  dense  and 
exceptionally  refractory.  If  platinum  can  be  vaporised, 


264  THE   EARTH'S    BEGINNING. 

there  is  not  much  difficulty  in  seeing  that  other 
elements  must  be  capable  of  being  vaporised  also.  In 
fact,  given  such  heat  as  is  found  abundantly  in  natural 
sources,  there  is  no  known  element,  or  combination 
of  elements,  which  will  not  assume  the  form  of  gas  or 
vapour  or  cloud. 

At  the  temperature  of  the  sun  a  drop  of  water 
would  be  forthwith  resolved  into  its  component  gases 
of  oxygen  and  hydrogen.  In  like  manner  a  piece  of 
chalk,  if  exposed  to  the  sun,  would  be  speedily  trans- 
formed; it  would  first  be  heated  red-hot  and  then 
white-hot;  it  is,  indeed,  white-hot  chalk  that  gives 
us  that  limelight  which  we  know  so  well.  But  the 
heat  of  the  sun  is  far  greater  than  the  temperature 
of  the  incandescent  lime.  The  lime  would  not  only  be 
heated  white-hot  by  contact  with  solar  heat,  but  still 
further  stages  would  be  reached.  It  would  suffer  de- 
composition. It  would  break  up  into  three  different 
elements :  there  would  be  the  metal  which  we  call 
calcium,  there  would  be  oxygen,  and  there  would  be 
carbon.  Owing  to  the  tremendous  temperature  of 
the  sun  the  metal  would  not  remain  in  the  metallic 
form;  it  would  not  be  even  in  a  liquid  form;  it 
would  become  a  gas.  The  elements  which  unite  to 
form  this  chalk  would  be  not  only  decomposed, 
but  they  would  be  vaporised.  What  is  thus  stated 
about  the  drop  of  water  and  the  chalk  may,  so  far 
as  we  know,  be  stated  equally  with  regard  to  any 
other  compounds.  It  matters  not  how  close  may  be 
the  chemical  association  in  which  the  elements  are 
joined:  no  matter  how  successfully  those  compounds 
may  resist  the  decomposition  under  the  conditions 
ordinarily  prevailing  on  earth,  they  have  to  yield 


THE    ASTRONOMER    AND    ARGON.  2(55 

under  the  overwhelming  trial  to  which  the  sun  would 
subject  them.  Though  there  are  many  elements  in 
the  solar  chemistry,  there  are  no  compounds.  At  the 
exalted  temperature  to  which  they  are  exposed  in  the 
sun  the  elements  are  indisposed  for  union  with  the 
other  elements  there  met  with,  and  which  are  at 
the  same  temperature.  In  these  circumstances,  they 
successfully  resist  all  alliances. 

Until  the  last  few  years  no  elements  were  known 
in  our  terrestrial  experience  which  possessed  at  ordinary 
temperatures  the  same  qualities  of  resolute  isolation 
which  all  elements  seem  to  display  at  extreme  tem- 
peratures. The  famous  discovery  of  argon,  and  of 
other  strange  gases  associated  with  argon  in  the  atmo- 
sphere and  elsewhere,  has  revealed,  to  the  astonishment 
of  chemists  and  to  the  great  extension  of  knowledge, 
that  we  have  with  us  here  elements  which  resist  all 
solicitations  to  enter  into  chemical  union  with  other 
substances.  It  is  doubtless  in  consequence  of  this 
absolute  refusal  to  unite  that,  in  spite  of  their  abun- 
dance and  their  wide  distribution,  these  elements  have 
eluded  detection  for  centuries.  To  the  astronomer 
argon  is  both  interesting  and  instructive.  It  shows  us 
an  element  which  possesses,  at  the  ordinary  tempera- 
tures of  the  surface  of  the  earth,  a  property  which  is 
true  of  all  elements  when  subjected  to  such  tempera- 
tures as  are  found  in  the  sun. 

Think  of  the  rocks  which  form  the  earth's  crust 
and  of  the  minerals  which  lie  far  below.  Think  of 
the  soil  which  lies  on  its  surface,  of  the  forests  which 
that  soil  supports,  and  the  crops  which  it  brings  forth. 
Think  of  the  waters  of  the  ocean,  and  the  ice  of  the 
Poles.  Think  of  the  objects  of  every  kind  on  this 


266  THE    EARTH'S    BEGINNING. 

globe.  Think  of  the  stone  walls  of  a  great  building, 
of  the  iron  used  to  give  it  strength,  of  the  slates 
which  cover  it,  and  of  the  timber  which  forms  its 
floors ;  think  of  the  innumerable  other  materials  which 
have  gone  towards  its  construction ;  think  even  of  the 
elementary  substances  which  go  to  form  the  bodies  of 
animals,  of  the  lime  in  their  bones,  and  of  the  carbon 
which  is  so  intimately  associated  with  life  itself. 
The  nebular  theory  declares  that  those  materials 
have  not  always  been  in  the  condition  in  which  we 
now  see  them  ;  that  there  was  a  time  in  which  they 
were  so  hot  that  they  were  not  in  the  solid  state ;  they 
were  not  even  in  the  fluid  state,  but  were  all  in  rolling 
volumes  of  glowing  vapour  which  formed  the  great 
primaeval  fire-cloud. 

We  must  understand  the  composite  nature  of  the 
primitive  fire-mist  from  which  our  solar  system  origi- 
nated. Let  me  illustrate  the  matter  thus:  We  shall 
suppose  that  a  heterogeneous  collection  of  substances  is 
brought  together,  the  items  of  which  may  be  somewhat 
as  follows :  let  there  be  many  tons  of  iron  and  barrels 
of  lime,  some  pieces  of  timber,  and  cargoes  of  flint ; 
let  there  be  lead  and  tin  and  zinc,  and  many  other 
metals,  from  which  copper  and  silver  and  several  of  the 
rarest  metals  must  not  be  excluded ;  let  there  be  in- 
numerable loads  of  clay,  which  shall  represent  aluminium 
and  silicon,  and  hogsheads  of  sea-water  to  supply 
oxygen,  hydrogen,  and  sodium.  There  should  be  also, 
I  need  hardly  add,  many  other  elements;  but  there 
is  no  occasion  to  mention  more;  indeed,  it  would  be 
impossible  to  give  a  list  which  would  be  complete. 

Suppose  that  this  diverse  material  is  submitted  to  a 
heat  as  intense  as  the  most  perfect  furnace  can  make  it. 


THE    GREAT   FIRE-MIST.  267 

Let  the  heat  be  indeed  as  great  as  that  which  we  can 
get  from  the  electric  arc,  or  even  greater  still.  Let  us 
suppose  this  heat  to  be  raised  to  such  a  point  that,  not 
only  have  the  most  refractory  metals  been  transformed 
into  vapour,  but  the  elements  which  were  closely  in 
combination  have  also  been  rent  asunder.  This  we 
know  will  happen  when  compound  substances  are  raised 
to  a  very  high  temperature.  We  shall  suppose  that  the 
heat  has  been  sufficient  to  separate  each  particle  of 
water  into  its  constituent  atoms  of  oxygen  and  hydrogen ; 
we  shall  suppose  that  the  heat  has  been  sufficient  to 
decompose  even  lime  itself  into  its  constituent  parts,  and 
exhibit  them  in  the  form  of  vapour.  The  heat  is  to  be 
so  great  that  even  carbon  itself,  the  most  refractory  of 
substances,  has  had  to  yield,  so  that  after  passing 
through  a  stage  of  dazzling  incandescence  it  has  melted 
and  ultimately  dissolved  into  vapour.  Next  let  us 
suppose  that  these  several  vapours  are  blended,  though 
we  need  not  assume  that  the  separate  elements  are 
diffused  uniformly  throughout  all  parts  of  the  cloud. 
Let  us  suppose  that  these  bodies,  which  contributed  to 
form  the  nebula,  have  been  employed  in  amounts,  not  to 
be  measured  in  tons,  or  in  hundreds  of  tons,  but  in 
a  thousand  millions  of  millions  of  millions  of  millions 
of  tons.  Let  the  mass  of  vapour  thus  arising  be 
expanded  freely  through  open  space.  Let  it  extend 
over  a  region  which  is  to  measure  hundreds  of  thousands 
of  millions  of  miles  in  length  and  breadth  and  depth. 
Then  the  doctrine  of  the  earth's  beginning,  which  we  are 
striving  to  unfold  in  these  lectures,  declares  that  in  a 
fire-mist  such  as  is  here  outlined  the  solar  system  had 
its  origin. 

Yarious  objections  may   occur    to  the    thoughtful 


268  THE   EARTH'S   BEGINNING. 

reader  when  asked  to  accept  such  statements.  We  must 
do  our  best  to  meet  these  objections.  The  evidence  we 
submit  must  be  of  an  indirect  or  circumstantial  kind. 
Direct  testimony  on  such  a  subject  is  from  the  nature  of 
the  case  impossible.  The  actual  fire-mist  in  which  our 
system  had  its  origin  is  a  mist  no  longer.  The  material 
that  forms  the  solid  earth  beneath  our  feet  did  once,  we 
verily  believe,  float  in  the  great  primaeval  fire-mist.  Of 
course  we  cannot  show  you  that  mist.  Darwin  could 
not  show  the  original  monkeys  from  which  it  would 
seem  the  human  race  has  descended ;  none  the  less  do 
most  of  us  believe  that  our  descent  has  really  taken  the 
line  that  Darwin's  theory  indicates. 

In  connection  with  this  subject,  as  with  most  others, 
it  is  easy  to  ask  questions  which,  I  think  we  may  say,  no 
one  can  answer  with  any  confidence.  It  may,  for 
instance,  be  asked  how  this  vast  fire-mist  came  into 
existence.  If  it  arose  from  heat,  how  did  that  heat 
happen  to  be  present?  Why  was  all  the  material  in 
the  state  of  vapour  ?  What,  in  short,  was  the  origin  of 
that  great  primaeval  nebula  ?  Here  we  must  admit  that 
we  have  proposed  questions  to  which  it  is  impossible  for 
us  to  do  more  than  suggest  answers.  As  to  what 
brought  the  mist  into  existence,  as  to  whence  the 
materials  came,  and  as  to  whence  the  energy  was 
derived  which  has  been  gradually  expended  ever  since, 
we  do  not  know  anything,  and,  so  far  as  I  can  see,  we 
have  no  means  of  knowing.  Conjectures  on  the  subject 
are  not  wanting,  of  course,  and  in  a  later  chapter  we 
shall  discuss  what  may  be  said  on  this  matter. 

I  have  shown  you  to  some  extent  our  reasons  for 
believing  that  our  solar  system  did  originate  in  a  fire- 
mist.  And  even  if  we  are  not  able  to  explain  how 


THE    MOST   IMPORTANT   DISCOVERY.          26S 

the  mist  itself  arose,  yet  we  do  not  admit  that  our 
argument  as  to  the  origin  of  our  system  is  thereby 
invalidated.  That  such  a  fire-mist  as  the  solar  system 
required  did  once  exist,  must  surely  be  regarded  as 
not  at  all  improbable  so  long  as  we  can  point  to  the 
analogous  nebulae  or  fire-mists  which  exist  at  the 
present  moment,  and  which  we  see  with  our  telescopes. 
Many  of  these  are  millions  of  times  as  great  as  the 
comparatively  small  fire-mist  that  would  have  evolved 
into  our  solar  system. 

A  question  has  sometimes  been  asked  as  to  the 
most  important  discovery  in  astronomy  which  has 
been  made  in  the  century  that  has  just  closed. 
If,  by  the  most  important  discovery,  we  mean 
that  which  has  most  widely  extended  our  know- 
ledge of  the  Universe,  I  do  not  think  there  need  be 
much  hesitation  in  stating  the  answer.  It  seems  to 
me  beyond  doubt  that  the  most  astonishing  discovery 
of  the  last  century  in  regard  to  the  heavenly  bodies 
is  that  which  has  revealed  the  elementary  substances 
of  which  the  orbs  of  heaven  are  composed.  This  dis- 
covery is  the  more  interesting  and  instructive  because 
it  has  taught  us  that  the  materials  of  the  sun,  of  the 
stars,  and  of  the  nebulae  are  essentially  the  elements 
of  which  our  own  earth  is  formed,  and  with  which 
chemists  had  already  become  well  acquainted. 

We  know,  of  course,  that  this  earth,  no  matter 
how  various  may  be  the  rocks  and  minerals  which 
form  its  crust,  and  how  infinite  the  variety  of  objects, 
organic  and  inorganic,  which  diversify  its  surface,  is 
really  formed  from  different  combinations  of  about 
eighty  different  elements.  There  are  gases  like  oxygen 
and  hydrogen,  there  are  other  substances  like  carbon 


270  THE    EARTH'S    BEGINNING. 

and  sulphur,  and  there  are  metals  like  iron  and  copper. 
These  elements  are  sometimes  met  with  in  their  free 
or  uncombined  state,  like  oxygen  in  the  atmosphere, 
or  like  gold  in  Klondike.  More  frequently  they  are 
found  in  combination,  and  in  such  combinations  the 
characters  of  the  constituent  elements  are  some- 
times completely  transformed.  A  deadly  gas  and  a 
curious  metal,  which  burns  as  it  floats  on  water,  most 
certainly  renounce  their  special  characters  when  they 
unite  to  form  the  salt  on  our  breakfast- table.  Who 
would  have  guessed,  if  the  chemist  had  not  told 
him,  that  in  every  wheelbarrowful  of  ordinary  earth 
there  are  pounds  of  silvery  aluminium,  and  that  marble 
is  largely  composed  of  an  extremely  rare  metal,  which 
but  few  people  have  ever  seen  ? 

Until  the  middle  of  the  century  just  completed  it 
seemed  utterly  impossible  to  form  any  notion  as  to 
the  substances  actually  present  in  the  sun.  How 
could  anyone  possibly  discern  them  by  the  resources 
of  the  older  chemists  ?  It  might  well  have  been 
doubted  whether  the  elements  of  which  the  sun  was 
made  were  the  elements  of  which  our  earth  was  formed, 
and  with  which  ordinary  chemistry  had  made  us 
familiar.  Just  as  the  animals  and  plants  which  met 
the  gaze  of  the  discoverers  when  they  landed  in  the 
New  World  were  essentially  different  from  those  in 
the  Old  World,  so  it  might  have  been  supposed,  with 
good  share  of  reason,  that  this  great  solar  orb,  ninety- 
three  million  miles  distant,  would  be  composed  of 
elements  totally  different  from  those  with  which 
dwellers  on  the  earth  had  been  permitted  to  become 
acquainted. 

This   great  discovery  of  the   last   century  revealed 


THE    CHEMISTRY    OF    THE   SUN.  271 

to  us  the  character  of  the  elements  which  constitute 
the  sun.  It  also  added  the  astonishing  information 
that  they  are  essentially  the  same  elements  as  those 
of  which  our  earth  itself  and  all  which  it  contains 
are  formed. 

If  any  one  had  asked  in  the  early  years  of  the 
century  what  those  elements  were  which  entered  into 
the  composition  of  the  sun,  the  question  would  have 
been  deemed  a  silly  one ;  it  would  have  been  regarded 
as  hopelessly  beyond  the  possibility  of  solution,  and 
it  would  have  been  as  little  likely  to  receive  an  answer 
as  the  questions  people  sometimes  ask  now  as  to  the 
possible  inhabitants  on  Mars. 

But  about  the  middle  of  the  century  a  new  era 
dawned  ;  the  wonderful  method  of  spectroscopic  analysis 
was  discovered,  and  it  became  possible  to  examine  the 
chemistry  of  the  sun.  The  most  important  result  was 
to  show  that  the  elements  which  enter  into  the  com- 
position of  the  sun  are  the  same  elements  which  enter 
into  the  composition  of  the  earth.  The  student  of 
the  solar  chemistry  enjoys,  however,  one  advantage  over 
the  terrestrial  chemist,  if  it  be  an  advantage  to  have 
his  science  simplified  to  the  utmost  extent.  Chemistry 
would,  however,  lose  its  chief  interest  if  all  the  elements 
remained  as  obstinately  neutral  as  argon,  and  disdained 
alliance  with  all  other  elements.  It  would  seem  that 
those  elements  which  most  eagerly  enter  into  combina- 
tion here,  and  which  resist  with  such  vehemence  our 
efforts  to  divorce  them,  must  renounce  all  chemical 
union  when  exposed  to  the  tremendous  temperature 
of  the  sun. 

Those  elements  which  unite  with  the  utmost  eager- 
ness at  ordinary  temperatures,  seem  to  become  indifferent 


272  TEE   EARTH'S    BEGINNING. 

to  each  other  when  subjected  to  the  extremes  of  heat 
and  cold.  Potassium  unites  fiercely  with  oxygen  in  the 
most  familiar  of  all  chemical  experiments.  Potassium 
is  indeed  a  strange  metal,  for  it  is  of  such  small  density 
that  a  piece  cast  on  a  basin  of  water  will  float  like 
a  chip  of  wood.  It  has  such  avidity  for  oxygen  that 
it  will  decompose  the  water  to  wrench  the  molecules- 
of  "oxygen  from  those  of  hydrogen.  The  union  of  the 
metal  with  the  gas  generates  such  heat  that  the  strange 
substance  bursts  into  flame.  This  is  what  takes  place 
at  the  ordinary  temperatures  in  the  well-known  ex- 
periment of  the  chemical  lecture- table.  But  at  extreme 
temperatures  the  greed  of  potassium  for  oxygen  abates, 
if  it  does  not  vanish  altogether.  In  those  excessively 
low  temperatures  at  which  Professor  Dewar  experiments 
chemical  affinities  languish.  He  has  reduced  oxygen 
to  a  liquid,  and  he  tells  us  that  "a  berg  of  silvery 
potassium  might  float  for  ever  untarnished  on  an  ocean 
of  liquid  oxygen."  At  the  excessively  high  temperature 
of  the  electric  arc  the  oxygen  and  the  potassium,  whose 
union  has  been  accomplished  with  such  vehemence,, 
cease  to  possess  affinity,  and  they  separate  again. 

The  solar  chemistry  seems  to  know  no  combina- 
tion. If  a  drop  of  water  were  transferred  to  the  sun 
and  subjected  to  the  heat  of  the  solar  surface,  it  must 
immediately  undergo  decomposition.  That  which  was 
a  drop  of  water  here  would  not  remain  a  drop  of  water 
there ;  it  would  be  at  once  resolved  into  its  component 
elements  of  oxygen  and  hydrogen.  The  considerations 
just  given  greatly  simplify  the  search  for  the  particular- 
bodies  which  are  at  present  in  the  sun.  We  have  only 
to  test  for  the  presence  of  each  of  eighty  elements.  We 
have  not  to  take  account  of  the  thousands  ot  chemical 


THE   ELEMENTS   IN   THE   SUN,  273 

combinations  of  which  these  elements  are  susceptible 
under  terrestrial  conditions. 

We  are  specially  indebted  to  the  late  Professor 
Henry  Kowland,  of  Baltimore,  for  a  profound  study  of 
the  solar  spectrum.  In  his  great  work  he  enumerates 
thirty-six  elements  present  in  the  sun,  and  the  number 
may  be  increased  now  by  at  least  two.  Eight  elements 
he  classes  as  doubtful,  fifteen  are  set  down  as  absent 
from  the  solar  spectrum,  and  several  had  not  been  tried. 
Iron  stands  foremost  among  all  the  solar  elements,  so 
far  as  the  number  of  its  lines  are  concerned.  No  fewer 
than  2,000  lines  in  the  spectrum  of  the  sun  are  attributed 
to  this  element.  At  the  other  end  of  the  list  lead  is 
found.  There  is  only  one  line  apparently  due  to  this 
metal.  Carbon  is  represented  by  about  200  lines,  and 
calcium  by  about  75.  If,  however,  we  test  the  sig- 
nificance of  lines  not  by  their  number,  but  by  their 
intensity,  then  iron  no  longer  heads  the  list,  its  place 
being  taken  by  calcium  (Fig.  42).  Among  the  elements 
which  Rowland  sets  down  as  not  contributing  any  recog- 
nisable lines  to  the  solar  spectrum  we  may  mention 
arsenic  and  sulphur,  phosphorus,  mercury,  and  gold. 

Of  the  more  prominent  solar  elements  there  are  two 
or  three  of  such  special  importance  that  we  pause  to 
give  them  a  little  consideration.  Who  does  not  re- 
member the  delight  of  the  first  occasion  in  childhood 
when  he  was  permitted  to  peep  into  a  bird's-nest  and 
there  see  a  group  of  eggs,  often  so  exquisitely  marked  or 
so  delicately  tinted  ?  How  beautiful  they  seemed  as 
they  lay  in  their  cosy  receptacle  concealed  with  so  much 
cunning  !  Among  other  delightful  recollections  of  early 
youth  many  will  recall  a  ramble  by  the  sea-shore.  We 
may  suppose  the  tide  had  retreated,  and  with  other 


274  THE    EARTH'S    BEGINNING. 

objects  left  by  the  sea  on  the  gleaming  sand  a  little 
cowrie  shell  is  found.  How  enchanted  we  were  with  our 
prize !  How  we  looked  at  the  curious  marks  on  its  lips, 
and  the  inimitable  beauty  of  its  tints ! 

The  shell  of  the  hedge-sparrow  and  the  shell  cast  up 
by  the  sea  have  another  quality  in  common  besides  their 
beauty.  They  have  both  been  fabricated  from  the  same 
material.  Lime  is  of  course  the  substance  from  which 
the  bird,  by  some  subtle  art  of  physiology,  forms  those 
exquisite  walls  by  which  the  vital  part  of  the  egg  is 
protected.  The  soft  organism  that  once  dwelt  in  the 
cowrie  was  endowed  with  some  power  by  which  it 
extracted  from  the  waters  of  the  ocean  the  lime  with 
which  it  gradually  built  an  inimitable  shell.  Is  it  an 
exaggeration  to  say  that  this  particular  element  calcium, 
this  element  so  excessively  abundant  and  so  rarely  seen, 
seems  to  enjoy  some  peculiar  distinction  by  association 
with  exquisite  grace  and  beauty?  The  white  marble 
wrought  to  an  unparalleled  loveliness  by  the  genius  of  a 
Phidias  or  a  Canova  is  but  a  form  of  lime.  So  is  the 
ivory  on  which  the  Japanese  artist  works  with  such 
delicacy  and  refinement.  Whether  as  coral  in  a  Pacific 
island,  as  a  pearl  in  a  necklace  or  as  a  stone  in  the 
Parthenon,  lime  seems  often  privileged  to  form  the 
material  basis  of  beauty  in  nature  and  beauty  in  art. 

Though  lime  in  its  different  forms,  in  the  rocks  of  the 
earth  or  the  waters  of  the  ocean,  is  one  of  the  most 
ordinary  substances  met  with  on  our  globe,  yet  calcium, 
the  essential  element  which  goes  to  the  composition  of 
lime,  is,  as  we  have  already  said,  not  by  any  means  a 
familiar  body,  and  not  many  of  us,  I  imagine,  can  ever  have 
seen  it.  Chemistry  teaches  that  lime  is  the  result  of  a 
union  in  definite  proportions  between  oxygen  gas  and 


-     A    VERT   SHY  METAL.  275 

the  very  shy  metal,  calcium.  This  metal  is  never  found 
in  nature  unless  in  such  intimate  chemical  union  with 
some  other  element  like  oxygen  or  chlorine,  that  its 
characteristic  features  are  altogether  obscured,  and  would 
indeed  never  be  suspected  from  the  mere  appearance  of 
the  results  of  the  union.  To  see  the  metal  calcium  you 
must  visit  a  chemical  laboratory  where,  by  electrical 
decomposition  or  other  ingenious  process,  this  elusive 
element  can  be  induced  to  part  temporarily  from  its 
union  with  the  oxygen  or  other  body  for  which  it  has  so 
eager  an  affinity,  and  to  which  it  returns  with  such 
alacrity.  Though  calcium  is  certainly  a  metal,  it  is  very 
unlike  the  more  familiar  metals  such  as  gold  or  silver, 
copper  or  iron.  A  coin  might  conceivably  be  formed 
out  of  calcium,  but  it  would  have  no  stability  like  the 
coins  of  the  well-known  metals.  Calcium  has  such  an 
unconquerable  desire  to  unite  with  oxygen  that  the 
unstable  metal  will  speedily  grasp  from  the  surrounding 
air  the  vital  element.  Unless  special  precautions  are 
taken  to  withhold  from  the  calcium  the  air,  or  other 
source  from  whence  it  could  obtain  oxygen,  the  union 
will  most  certainly  take  place,  and  the  calcium  will 
resume  the  stable  form  of  lime.  Thus  it  happens  that 
though  this  earth  contains  incalculable  billions  of  tons 
of  calcium  in  its  various  combinations,  yet  calcium  itself 
is  almost  unknown  except  to  the  chemist. 

It  is  plain  that  calcium  plays  a  part  of  tremendous 
significance  on  this  earth.  I  do  not  say  that  it  is  the 
most  important  of  all  the  elements.  It  would  indeed 
seem  impossible  to  assign  that  distinction  to  any  par- 
ticular element.  Many  are,  of  course,  of  vital  importance, 
though  there  are,  no  doubt,  certain  of  the  rarer  elements 
with  which  this  earth  could  perhaps  dispense  without 


276 


THE   EARTH'S   BEGINNING. 


being  to  any  appreciable  extent  different  from  what  it  is 
at  present.  I  do  not  know  that  we  should  be  specially 
inconvenienced  or  feel  any  appreciable  want  unsatisfied, 
if,  let  us  say,  the  element  lanthanum  were  to  be  struck 
out  of  existence ;  and  there  are  perhaps  certain  other 


LULL 


Fig.  42. — THE  H.  AND  K.  LINES  IN  THE  PHOTOGRAPHIC  SOLAR 
SPECTRUM  (Higgs), 

rare  bodies  among  the  known   eighty  elements,  about 
which  the  same  remark  might  be  made. 

But  without  calcium  there  would  neither  be  fertile 
soil  for  plants  nor  bones  for  animals,  and  consequently 
a  world,  inhabited  in  the  same  manner  as  our  present 
globe,  would  be  clearly  impossible.  There  may  be  lowly 
organisms  on  this  earth  to  which  calcium  is  of  no 
appreciable  consequence,  and  it  is  of  course  conceivable 
that  a  world  of  living  types  could  be  constructed  with- 
out the  aid  of  that  particular  element  which  is  to  us 
so  indispensable.  But  a  world  without  calcium  would 
be  radically  different  from  that  world  which  we  know, 
so  that  we  are  disposed  to  feel  special  interest  in  the 
important  modern  discovery  that  this  same  element, 
calcium,  is  abundantly  distributed  throughout  the 
universe.  The  boldest  and  most  striking  features  in 


HELIUM.  277 

the  photograph  ot  the  solar  spectrum  are  those  due 
to  calcium  (Figs.  42  and  44). 

In  the  solar  spectrum  are  two  very  broad,  very  dark, 
and  very  conspicuous  lines,  known  as  H  and  K.  In 
every  photograph  of  that  portion  of  the  solar  spectrum 
which,  lying  beyond  the  extreme  violet,  is  invisible  to 
our  eyes,  though  intensely  active  on  the  photographic 
plate,  these  lines  stand  forth  so  boldly  as  to  arrest  the 
attention  more  than  any  other  features  of  the  spectrum. 
It  had  been  known  that  these  lines  were  due  to  calcium, 
but  there  were  certain  difficulties  connected  with  their 
interpretation.  Some  recent  beautiful  researches  by 
Sir  William  and  Lady  Huggins  have  cleared  away  all 
doubt.  It  is  now  certain  that  the  presence  of  these 
lines  in  the  spectrum  demonstrates  that  that  remarkable 
element  which  is  the  essential  feature  of  lime  on  this 
earth  is  also  found  in  the  sun.  We  have  also  to 
note  that  these  same  lines  have  been  detected  in  the 
photographic  spectra  of  many  other  bodies  in  widely 
different  regions  of  space.  Thus  we  establish  the 
interesting  result  that  this  particular  element  which 
plays  a  part  so  remarkable  on  our  earth  is  not  restricted 
to  our  globe,  but  is  diffused  far  and  wide  throughout 
the  universe. 

Perhaps  the  most  astonishing  discovery  made  in 
modern  times  about  the  sun  is  connected  with  the 
wonderful  element,  helium.  So  long  ago  as  1868  Sir 
Norman  Lockyer  discovered,  during  an  eclipse,  that 
the  light  of  the  sun  contained  evidence  of  the  presence 
in  that  orb  of  some  element  which  was  then  totally 
unknown  to  chemists.  This  new  body  was  not  un- 
naturally named  the  sun-element,  or  helium.  But  more 
than  a  quarter  of  a  century  had  to  elapse  before  any 
19 


278  THE    EARTH'S    BEGINNING. 

chemist  could  enjoy  the  opportunity  of  experimen ting- 
directly  upon  helium.  No  labour  could  prepare  the 
smallest  particle  of  this  substance,  no  money  could 
purchase  it,  for  at  that  time  no  specimen  of  the  element 
was  known  to  exist  nearer  than  the  sun,  ninety-three 
million  miles  distant.  But  in  1895  an  astonishing 
discovery  was  made  by  Professor  Ramsay.  He  was 
examining  a  rare  piece  of  mineral  from  Norway.  From 
this  mineral,  clevite,  the  Professor  extracted  a  little 
gas  which  was  to  him  and  to  all  other  chemists  quite 
unknown.  But  on  applying  the  spectroscope  to  examine 
the  character  of  the  light  which  this  gas  emitted  when 
submitted  to  the  electric  current,  it  yielded,  to  their 
amazement,  the  characteristic  light  of  helium.  Thus 
was  the  sun-element  at  last  shown  to  be  a  terrestrial 
body,  though  no  doubt  a  rare  one.  The  circum- 
stances that  I  have  mentioned  make  helium  for  ever 
famous  among  the  constituents  of  the  universe.  It  will 
never  be  forgotten  that  though  from  henceforth  it 
may  be  regarded  as  a  terrestrial  body,  yet  it  was  first 
discovered,  not  in  the  earth  beneath  our  feet,  but  in 
the  far-distant  sun. 

In  a  previous  picture  (Fig.  14)  we  showed  a  photograph 
of  a  part  of  the  sun's  surface  ;  this  striking  view  displays 
those  glowing  clouds  from  which  the  sun  dispenses 
its  light  and  heat.  These  clouds  form  a  comparatively 
thin  stratum  around  the  sun,  the  interior  of  which  is 
very  much  darker.  The  layer  of  clouds  is  so  thin 
that  it  may  perhaps  be  likened  to  the  delicate  skin 
of  a  peach  in  comparison  with  the  luscious  interior 
It  is  in  these  dazzling  white  clouds  that  we  find  the 
source  of  the  sun's  brightness.  Were  those  cloud 
removed,  though  the  sun's  diameter  would  not  be 


THE    FIRE-CLOUDS    IN   THE    SUN.  279 

appreciably  reduced,  yet  its  unparalleled  lustre  would 
be  at  once  lessened.  We  use  the  expression  "  clouds  " 
in  speaking  of  these  objects,  for  clouds  they  certainly 
are,  in  the  sense  of  being  aggregates  of  innumerable 
myriads  of  minute  beads  of  some  substance ;  but  those 
solar  clouds  are  very  unlike  the  clouds  of  our  own 
sky,  in  so  far  as  the  material  of  which  they  are  made 
is  concerned.  The  solar  clouds  are  not  little  beads 
of  water;  they  are  little  beads  of  white  hot  material 
so  dazzlingly  bright  as  to  radiate  forth  the  characteristic 
brilliance  and  splendour  of  the  sun.  The  solar  clouds 
drift  to  and  fro ;  they  are  occasionally  the  sport  of 
terrific  hurricanes;  they  are  sometimes  driven  away 
from  limited  areas,  and  in  their  absence  we  see  merely 
the  black  interior  of  the  solar  globe,  which  we  call 
a  sun-spot.  Now  comes  the  important  question  as  to 
the  material  present  in  these  clouds  which  confers 
on  the  sun  its  ability  to  radiate  forth  such  abundant 
light  and  heat. 

The  profound  truth  already  stated,  that  the  solar 
elements  are  the  same  as  the  terrestrial  elements, 
greatly  simplifies  the  search  for  that  particular  element 
which  forms  those  solar  clouds.  As  the  sun  is  made 
of  substances  already  known  to  us  by  terrestrial 
chemistry,  and  as  there  are  no  chemical  compounds 
to  embarrass  us,  the  choice  of  the  possible  constituents 
of  those  solar  clouds  becomes  narrowed  to  the  list  of 
elements  experimented  on  in  our  laboratories. 

We  owe  to  Dr.  G.  Johnstone  Stoney,  F.R.S.,  the 
discovery  of  the  particular  element  which  forms  those 
fire-clouds  in  the  sun,  and  confers  on  the  presiding  body 
of  the  solar  system  the  power  of  being  so  useful  to  the 
planets  which  owe  it  allegiance.  Carbon  is  the  element 


280  THE    EARTH'S    BEGINNING. 

in  question.  I  need  hardly  add  that  carbon  is  well 
known  as  one  of  the  most  commonplace  and  one  of 
the  most  remarkable  substances  in  Nature.  A  piece  of 
coke  differs  from  a  piece  of  pure  carbon  only  by  the  ash 
which  the  coke  leaves  behind  when  burned.  Timber  is 
principally  composed  of  this  same  element,  and  when 
the  timber  is  transformed  into  charcoal  but  little  more 
than  the  carbon  remains.  Carbon  is  indeed  every- 
where present.  It  is,  as  we  have  mentioned,  one  of  the 
elements  which  enter  into  the  composition  of  a  piece 
of  chalk.  Carbon  is  in  the  earth  beneath  our  feet ;  it  is 
in  the  air  above  us.  Carbon  is  one  of  the  chief  in- 
gredients in  our  food,  and  it  is  by  carbon  that  the  heat 
of  the  body  is  sustained.  Indeed,  this  remarkable 
element  is  intimately  connected  with  life  in  every  phase. 
Every  organic  substance  contains  carbon,  and  it  courses 
with  the  blood  in  our  veins.  It  assumes  the  widest 
variety  of  forms,  renders  the  greatest  diversity  of  services, 
and  appears  in  the  most  widely  different  places.  Carbon 
is  indeed  of  a  protean  character,  and  there  is  a  beautiful 
symbol  of  the  unique  position  which  it  occupies  in  the 
scheme  of  Nature  (Fig.  43).  Carbon  is  associated  not 
alone  with  articles  of  daily  utility  and  of  plenteous 
abundance,  but  it  is  carbon  which  forms  the  most  ex- 
quisite gems  "  of  purest  ray  serene."  The  diamond  is,  of 
course,  merely  a  specimen  of  carbon  of  absolute  purity 
and  in  crystalline  form.  Great  as  is  the  importance  of 
carbon  on  this  earth,  it  is  spread  far  more  widely;  it  is 
not  confined  merely  to  the  earth,  for  carbon  abounds  on 
other  bodies  in  space.  The  most  important  functions  of 
carbon  in  the  universe  are  not  those  it  renders  on  this 
earth.  It  was  shown  by  Dr.  Stoney  that  this  same  wonder- 
ful substance  is  indeed  a  solar  element  of  vast  utility.  It 


THE    USEFULNESS    OF   CARBON.  281 

is  carbon  which  forms  the  glowing  solar  clouds  to  which 
our  very  life  owes  its  origin. 

In  the  incandescent  lamp  the  brilliant  light  is  pro- 
duced by  a  glowing  filament  of  carbon,  and  one  reason 
why  we  employ  this  element  in  the  electric  lamp,  in- 
stead of  any  other,  may  be  easily  stated.  If  we  tried  to 
make  one  of  these  lamps  with  an  iron  wire,  we  should  find 
that  when  the  electric  current  is  turned  on  and  begins 
to  flow  through  the  wire,  the  wire  will,  in  accordance 
with  a  well-known  law,  become  warm,  then  hot,  red -hot, 
and  white-hot ;  but  even  when  white-hot  the  wire  will 
not  glow  with  the  brightness  that  we  expect  from  one 
of  these  lamps.  Ere  a  sufficient  temperature  can  be 
reached  the  iron  will  have  yielded,  it  will  have  melted 
into  drops  of  liquid,  continuity  will  be  broken,  the  circuit 
will  be  interrupted,  and  the  lamp  destroyed.  We  should 
not  have  been  much  more  successful  if  instead  of  iron  we 
had  tried  any  other  metal.  Even  a  platinum  wire,  though 
it  will  admit  of  being  raised  to  a  much  higher  temperature 
than  a  wire  of  iron  or  a  wire  of  steel,  cannot  remain  in 
the  solid  condition  at  the  temperature  which  would  be 
necessary  if  the  requisite  incandescence  is  to  be  produced. 

There  is  no  known  metal,  and  perhaps  no  substance 
whatever,  which  has  so  high  a  temperature  of  fusion  as 
carbon.  A  filament  of  carbon,  alone  among  the  avail- 
able elements,  will  remain  continuous  and  unfused  while 
transmitting  a  current  intense  enough  to  produce  that 
dazzling  brilliance  which  is  expected  from  the  incandes- 
cent lamp.  This  is  the  reason  why  this  particular 
element  carbon  is  an  indispensable  material  for  the 
electrician. 

Modern  research  has  now  demonstrated  that  just  as 
we  employ  carbon  as  the  immediate  agent  for  producing 


282  THE    EARTH'S   BEGINNING. 

our  beautiful  artificial  light,  so  the  sun  uses  precisely 
the  same  element  as  the  agent  of  its  light  and  heat- 
giving  power.  In  the  extraordinary  fervour  which 
prevails  in  the  interior  of  the  sun  all  substances  of  every 
description  must  submit  to  be  melted,  nay,  even  to  be 
driven  into  vapour.  An  iron  poker,  for  instance,  would 
vanish  into  iron  vapour  if  submitted  to  this  appalling 
solar  furnace.  Even  carbon  itself  is  unable  to  remain 
solid  when  subjected  to  the  intense  heat  prevailing  in 
the  inner  parts  of  the  sun.  At  that  heat  carbon  must 
assume  the  form  of  gas  or  vapour,  just  as  iron  or  the 
other  substances  which  yield  more  readily  to  the  appli- 
cation of  heat. 

By  the  help  of  a  simple  experiment  we  may  illus- 
trate the  significance  of  the  carbon  vapours  in  the 
solar  economy.  Let  us  take  a  Bunsen  burner,  in 
which  the  air  and  gas  are  freely  mingled  before  they 
enter  into  combustion.  If  the  air  and  the  gas  be  pro- 
perly proportioned,  the  combustion  is  so  perfect  that 
though  a  great  deal  of  heat  is  produced  there  is  but 
little  light.  The  gas  burned  in  this  experiment  ought  to 
be  the  ordinary  gas  of  our  mains,  which  depends  for  its 
illuminating  power  on  the  circumstance  that  the  hydro- 
gen, of  which  the  gas  is  chiefly  composed,  is  largely 
charged  with  carbon.  The  illuminating  power  of  the 
gas  may  indeed  be  measured  by  its  available  richness 
in  carbon.  As  it  enters  the  burner  the  carbon  is  itself 
in  a  gaseous  form.  This  is  not,  of  course,  on  account  of 
a  high  temperature.  The  carbon  of  the  coal-gas  is  in 
chemical  union  with  hydrogen,  and  the  result  is  in  the 
form  of  invisible  gases.  It  is  these  composite  gases, 
blended  with  large  volumes  of  ordinary  hydrogen,  which 
form  the  illuminating  gas  of  our  mains. 


THE    BUNS  EN  BURNER.  283 

In  the  Bunsen  burner  the  admission  of  a  proper 
proportion  of  air,  which  becomes  thoroughly  mixed 
with  the  coal  gas,  produces  perfect  combustion.  In  the 
act  of  burning,  the  oxygen  of  the  air  unites  immediately 
with  the  gas;  it  combines  with  the  hydrogen  to  form 
watery  vapour,  and  it  combines  with  the  carbon  to 
form  gases  which  are  the  well-understood  products 

O  •!• 

of  combustion. 

Suppose,  now,  we  cut  off  the  supply  of  air  from 
the  Bunsen  burner,  which  can  be  done  in  a  moment 
by  placing  the  hand  over  the  ring  of  holes  at  the  bottom 
at  which  the  air  is  admitted.  Immediately  a  change 
takes  place  in  the  combustion.  In  place  of  the  steady, 
hardly  visible,  but  intensely  hot  flame  which  we  had 
before,  we  have  now  a  very  much  larger  flame  which 
makes  a  bright  and  flickering  flare  that  lights  up  the 
room.  If  we  re-admit  the  air  at  the  bottom  of  the 
burner  the  light  goes  down  instantly ;  the  small,  pale 
flame  replaces  it,  and  again  the  perfect  combustion 
gives  out  intense  heat  at  the  expense  of  the  light. 

The  remarkable  change  in  the  character  of  a  gas- 
flame  produced  by  admitting  air  to  mix  with  the  gas 
before  combustion  is,  of  course,  easily  explained.  The 
chemical  action  takes  place  with  much  greater  facility 
under  these  circumstances.  The  union  of  the  carbon 
in  the  coal  gas  with  the  oxygen  then  takes  place  so 
thoroughly  and  instantaneously  that  the  carbon  never 
seems  to  have  abandoned  the  gaseous  form  even  for 
a  moment  in  the  course  of  the  transformation.  But 
in  the  case  where  air  is  not  permitted  to  mingle  with 
the  gas,  the  supply  of  oxygen  to  unite  with  the  in- 
candescent gases  can  only  be  obtained  from  the  exterior 
of  the  flame.  The  consequence  is  that  the  glowing 


284  THE    EARTH'S   BEGINNING. 

gas  charged  with  carbon  vapour  is  chilled  to  some 
extent  by  contact  with  the  cold  air.  It  therefore  seems 
as  if  the  union  of  the  hydrogen  with  the  oxygen  per- 
mitted the  particles  of  carbon  in  the  flame  to  resume 
their  solid  form  for  a  moment.  But  in  that  solid  form 
these  particles,  being  at  a  high  temperature,  have  a 
wonderful  efficiency  for  radiation,  and  consequently 
brilliance  is  conferred  upon  the  light.  Most  of  the 
particles  of  carbon  speedily  unite  with  the  surrounding 
oxygen,  and  re-enter  the  gaseous  state  in  a  different 
combination.  Some  of  them,  however,  may  escape  this 
fate,  in  which  case  they  assume  the  undesirable  form 
of  smoke.  The  Bunsen  lamp  can  thus  be  made  to 
give  an  illustration  of  the  fact  that  when  carbon  vapours 
receive  a  chill,  the  immediate  effect  of  the  chill  is  to 
transform  the  carbon  from  the  gaseous  form  to  myriads 
of  particles  in  the  liquid,  or  more  probably  in  the  solid 
form.  In  the  latter  state  the  carbon  possesses  a  power 
of  radiation  greatly  in  excess  of  that  which  it  possessed 
in  the  gaseous  state,  even  though  the  gas  may  have 
been  at  a  much  higher  temperature  than  the  white- 
hot  solid  particles. 

We  can  now  apply  these  principles  to  the  explana- 
tion of  the  marvellous  radiation  of  light  and  heat  from 
the  great  orb  of  day.  The  buoyancy  of  the  carbon 
vapours  is  one  of  their  most  remarkable  characteristics  ; 
they  tend  to  soar  upwards  through  the  solar  atmosphere 
until  they  attain  an  elevation  considerably  over  that 
of  many  of  the  other  materials  in  the  heated  vapours 
surrounding  the  great  luminary.  We  may  illustrate 
what  happens  to  these  carbon  vapours  by  considering 
the  analogous  case  presented  in  the  formation  of  ordinary 
clouds  in  our  own  skies.  It  is  true,  no  doubt,  that 


CLOUDS   AND    STEAM.  285 

terrestrial  clouds  are  composed  of  material  very  different 
from  that  which  enters  into  the  solar  clouds.  Terres- 
trial clouds  of  course  arise  in  this  way ;  the  generous 
warmth  of  the  sun  evaporates  water  from  the  great 
oceans,  and  transforms  it  into  vapour.  This  vapour 
ascends  through  our  atmosphere,  not  at  first  as  a  visible 
cloud,  but  in  the  form  of  an  invisible  vapour.  It  is 
gradually  diffused  throughout  the  upper  air,  until  at 
last  particles  of  water,  but  recently  withdrawn  from  the 
oceans,  attain  an  altitude  of  a  mile  or  more  above  the 
surface  of  the  earth.  A  transformation  then  awaits 
this  aqueous  vapour.  In  the  coldness  of  those  elevated 
regions  the  water  can  no  longer  remain  in  the  form  of 
vapour.  The  laws  of  heat  require  that  it  shall  revert 
to  the  liquid  state.  In  obedience  to  this  law  the  vapour 
collects  into  liquid  beads,  and  it  is  these  liquid  beads, 
associated  in  countless  myriads,  which  form  the  clouds 
we  know  so  well.  The  same  phenomenon  of  cloud- 
production  is  witnessed  on  a  smaller  scale  in  the  form- 
ation of  the  visible  puffs  which  issue  from  the  funnel 
of  a  locomotive.  We  generally  describe  these  rolling 
white  volumes  as  steam ;  but  this  language  is  hardly 
correct.  Steam,  properly  so  called,  is  truly  as  invisible 
as  the  air  itself;  it  is  only  after  the  steam  has  done 
its  work  and  is  discharged  into  the  atmosphere,  and 
there  receives  a  chill,  that  it  becomes  suddenly  trans- 
formed from  the  purely  gaseous  state  into  clustering 
masses  of  microscopic  spheres  of  water,  and  thus 
becomes  visible. 

We  can  now  understand  the  transformation  of  these 
buoyant  carbon  vapours  which  soar  upwards  in  the  sun. 
They  attain  an  elevation  at  which  the  fearful  intensity  of 
the  solar  heat  has  been  so  far  abated  by  the  cold  of 


286  THE    EARTH'S   BEGINNING. 

outer  space  that  the  carbon  gas  is  not  permitted  to 
remain  any  longer  in  the  form  of  gas  ;  it  must  return 
to  the  liquid  or  to  the  solid  state.  In  the  first  stage 
on  this  return  the  carbon  gas  becomes  transformed, 
just  in  the  same  way  as  watery  vapour  ascending  from 
the  earth  becomes  transformed  into  the  fleecy  cloud. 
Under  the  influence  of  its  fall  in  temperature  the 
carbon  vapour  collects  into  a  clustering  host  of  little 
beads  of  carbon.  This  is  the  origin  of  the  glorious 
solar  clouds.  Each  particle  of  carbon  in  that  magnifi- 
cent radiant  surface  has  a  temperature,  and  con- 
sequently a  power  of  radiation,  probably  exceeding 
that  with  which  the  filament  of  carbon  glows  in  the 
incandescent  electric  arc.  When  we  consider  that 
millions  of  millions  of  square  miles  on  our  luminary 
are  covered  with  clouds,  of  which  every  particle  is  so 
intensely  bright,  we  shall  perhaps  be  able  to  form 
some  idea  of  that  inimitable  splendour  which  even 
across  the  awful  gulf  of  ninety- three  million  miles 
transmits  the  indescribable  glory  of  daylight. 

We  are  perhaps  at  present  living  rather  too  close  to 
the  period  itself  to  be  able  to  appreciate  to  its  full  extent 
the  greatness  of  that  characteristic  discovery  made  in 
astronomy  during  the  century  just  closed,  to  which 
the  present  chapter  relates.  In  the  early  part  of  the  last 
century  it  might  have  been  said — indeed,  by  a  certain 
very  distinguished  philosopher  it  actually  was  said — that 
a  limit  could  be  laid  down  bounding  the  possibilities  of 
our  knowledge  of  the  heavenly  bodies.  It  was  admitted 
that  we  might  study  the  movements  of  the  different 
orbs  in  vastly  greater  detail  than  had  been  hitherto 
attempted,  and  that  we  might  calculate  the  forces  to 
which  those  orbs  were  submitted.  With  the  help  of 


THE    SUPPOSED   LIMIT   TO    ASTRONOMY.      287 

mathematical    analysis   we    might    pursue   the    conse- 
quences of  these  forces  to  their  remote  ramifications ; 
we    might    determine    where    the    various    orbs   were 
situated  at  illimitably  remote  periods  in  the  past.     We 
might  calculate  the  positions  which  they  shall  attain  at 
epochs  to  be  reached  in  the  illimitably  remote  future ; 
we  might  discover  innumerable  new  stars  and  worlds ; 
and  we  might  map  down  and  survey  the  distant  parts  of 
the  universe.    We  might  even  sound  the  depths  of  space 
and  determine  the  distances  of  the  more  remote  celestial 
bodies,  much   more   distant  than   any  of  those  which 
have  already  yielded  their  secrets;  we  might  measure 
the  dimensions   of  those   bodies   and   determine   their 
weights ;  we  might  add  scores  or  hundreds  to  the  list  of 
the  known  planets  ;  we  might  multiply  many  times  the 
number  of  known  nebulae    and  star-clusters ;  we  might 
make  measurements  of  many  thousands  of  double  stars ; 
we  might  essay  the  sublime  task  of  forming  an  inventory 
of  the  stars  of  the  universe  and  compiling  a  catalogue  in 
which  the  stars  and  their  positions  would  be  recorded  in 
their  millions;  but,  said  the  philosopher  to  whom  I  have 
referred,  though   you   might   accomplish   all   this,   and 
much  more  in  the  same  direction,  yet  there  is  a  well- 
marked  limit  to  your  possible  achievements ;  you  can, 
he  said,  never  expect  to  discover  the  actual  chemical 
elements  of  which  the  heavenly  bodies  are  composed. 
Nobody  could  dispute  the  reasonableness  of  this  state- 
ment at  the  time  he  made  it ;  indeed,  it  seemed  to  be  a 
necessary  deduction  from   our  knowledge   of  the   arts 
of  chemistry,  as  those  arts  were  understood  before  the 
middle  of  the  last  century. 

In  the  prosecution   of  his  researches  by  the  older 
method,  the  chemist  could  no  doubt  discover  the  different 


288  THE    EARTH'S    BEGINNING. 

elements  of  which  the  body  was  formed.  That  is  to  say, 
his  art  enabled  him  to  accomplish  this  task,  provided  one 
very  essential  and  fundamental  condition  could  be  com- 
plied with.  However  accomplished  the  chemist  of  fifty 
years  ago  might  have  been,  he  would  assuredly  have 
thought  that  he  was  being  mocked  if  asked  to  determine 
the  composition  of  a  body  which  was  93,000,000  miles 
away  from  him.  The  very  idea  of  forming  an  analysis 
under  such  conditions  would  have  been  scouted  as  prepos- 
terous. He  would  naturally  ask  that  a  specimen  of  the 
body  should  be  delivered  into  his  hands,  a  specimen  which 
he  could  take  into  his  laboratory,  pulverise  in  his  mortars, 
place  in  his  test-tubes,  treat  with  his  re-agents,  or 
examine  with  his  blowpipe.  Only  by  such  methods 
was  it  then  thought  possible  to  obtain  an  analysis  and 
discover  the  elements  from  which  any  given  substance 
was  formed. 

For  in  the  early  part  of  this  century  the  splendid 
method  of  spectrum  analysis,  that  method  which  has 
revealed  to  us  so  many  of  the  secrets  of  Nature,  had  not 
yet  come  into  being.  When  that  memorable  event  took 
place  it  was  at  once  perceived  that  the  spectroscope 
required  no  actual  contact  with  the  object  to  be  tested, 
but  only  asked  to  receive  some  of  the  rays  of  light 
which  that  object  dispersed  when  sufficiently  heated. 
It  was  obvious  that  this  new  method  must  be  capable  of 
an  enormously  enlarged  application.  The  flame  pro- 
ducing the  vapour  might  be  at  one  end  of  the  room, 
while  the  spectroscope  testing  the  elements  in  that 
vapour  might  be  at  the  other  end.  This  new  and 
beautiful  optical  instrument  could  analyse  an  object 
at  a  distance  of  a  hundred  feet.  But  if  applicable  at  a 
distance  of  a  hundred  feet,  why  not  at  a  hundred  yards, 


INVISIBLE    SPECTRA    ANALYSED.  289 

or  a  hundred  miles,  or  a  hundred  million  miles  ?  Why 
might  the  method  not  be  used  if  the  source  of  light 
were  as  far  as  the  sun,  or  as  far  as  a  star,  or  even 
as  far  as  the  remotest  nebula,  whose  faint  gleam 
on  the  sky  is  all  that  the  mightiest  telescope  can 
show. 

Presently  another  great  advance  was  recorded.  As 
the  study  of  this  subject  progressed,  it  was  soon  found 
that  a  spectrum  visible  to  the  human  eye  was  not 
always  indispensable  for  the  success  of  the  analysis. 
The  photographic  plate,  which  so  frequently  replaces 
the  eye  in  other  classes  of  observation,  has  also  been 
used  to  replace  the  eye  in  the  use  of  the  spectroscope. 
A  picture  has  thus  been  obtained  showing  the  charac- 
teristic lines  in  the  spectrum  of  a  celestial  object.  That 
object  may  have  been  sunk  in  space  to  a  distance 
so  tremendous  that  even  though  the  light  travelled  at  a 
pace  sufficient  to  complete  seven  circuits  of  our  earth  in 
each  second  of  time,  yet  the  rays  from  the  object  in 
question  may  have  been  travelling  for  centuries  before 
they  reached  our  instrument. 

However  the  rays  of  light  may  have  become 
weakened  in  the  course  of  that  journey,  they  still  faith- 
fully preserve  the  credentials  of  their  origin.  At  last 
the  light  is  decomposed  in  the  spectroscope,  and  the 
several  rays,  which  have  been  so  closely  commingled 
in  their  long  voyage  of  myriads  of  miles,  are  now  for 
the  first  time  forced  to  pursue  different  tracks ;  they 
thus  reach  their  different  destinations  on  the  photo- 
graphic plate,  and  they  there  engrave  their  characteristic 
inscriptions.  Nature  in  this  operation  imparts  for  our 
instruction  a  message  which  it  is  our  business  to  in- 
terpret. It  is  true  that  these  inscriptions  are  not 


290 


THE   EARTH'S    BEGINNING. 


Fig.  43. — SPECTRUM  OF  COMET  SHOWING  CARBON  LINES. 
(Sir  W.  Huggins,  K.C.B.) 

always  easily  deciphered;  many  of  them  have  not  yet 
been  understood.  A  portion  of  the  solar  spectrum 
showing  many  of  the  lines  in  the  visible  region  is 
represented  in  the  accompanying  plate. 

Considering  the  insignificance  of  our  earth  when 
viewed  in  comparison  with  the  millions  of  other  orbs 
in  the  universe,  considering  also  the  stupendous  dis- 
tances by  which  the  earth  is  separated  from  innumerable 
globes  which  are  very  much  greater,  it  is  certainly 
not  a  little  astonishing  to  learn  that  the  elements  from 
which  the  various  bodies  in  the  universe  have  been 
composed  are  practically  the  same  elements  as  those 
of  which  our  earth  is  built.  Is  not  this  a  weighty 
piece  of  evidence  in  favour  of  the  theory  that  earth, 
sun,  and  planets  are  all  portions  of  the  same  primaeval 
nebula  in  which  these  elements  were  blended  ? 

We  do  not,  of  course,  mean  to  affirm  that  the 
great  primaeval  nebula  was  homogeneous  throughout  its 
vast  extent.  The  waters  of  ocean  are  not  strictly 
the  same  in  all  places;  even  the  atmosphere  is  not 


ELEMENTS   IN   THE   NEBULA.  291 


Fig.  44.— SPECTRUM  OP  SUN  DURING  ECLIPSE.     THE  Two 
CHIEF  LINES  ARE  DUE  TO  CALCIUM. 

(Ever  shed.) 

absolutely  uniform.  Nature  does  not  like  homogeneity. 
The  original  nebula,  we  may  well  believe,  was  irregular 
in  form,  and  denser  in  some  places  than  in  others. 
We  do  not  suppose  that  if  we  could  procure  a  sample 
of  nebula  in  one  place  and  another  sample  from  the 
same  nebula,  but  in  a  different  place,  say  a  hundred 
million  miles  distant,  the  two  would  show  an  identity 
of  chemical  composition;  two  samples  of  rock  from 
different  parts  of  the  same  quarry  will  not  always  be 
identical.  But  we  may  be  assured  that,  in  general, 
whatever  elements  are  present  in  the  nebula  will  be 
widely  dispersed  through  its  extent.  If  from  different 
parts  ol  the  nebula  two  globes  are  formed  by  conden- 
sation, though  we  should  not  affirm,  and  though  in 
fact  we  could  not  believe,  that  those  globes  would  be 
of  identical  composition,  yet  we  should  reasonably 
expect  that  the  elementary  bodies  which  entered  into 
their  composition  would  be  in  substantial  agreement. 
If  one  element,  say  iron,  was  abundant  in  one  globe, 
we  should  expect  that  iron  would  not  be  absent  from 
the  other.  Thus  the  elements  represented  in  one 


292  THE   EARTH'S    BEGINNING. 

body  should  be  essentially  those  which  were  represented 
in  the  other. 

It  is  obvious  that  if  the  sun  and  the  earth— to 
confine  our  attention  solely  to  those  two  bodies — had 
originated  from  the  primaeval  nebula,  they  would  bear 
with  them,  as  a  mark  of  their  common  origin,  a 
resemblance  in  the  elementary  bodies  of  which  they 
were  composed.  When  Laplace  framed  his  theory, 
he  had  not,  he  could  not  have  had,  the  slightest  notion 
as  to  the  particular  elements  in  the  sun.  For  anything 
he  could  tell,  those  elements  might  be  absolutely 
different  from  the  elements  in  the  earth.  Yet,  even 
without  information  on  this  critical  point,  the  evidence 
for  the  nebular  theory  appeared  to  him  so  cogent 
that  he  gave  it  the  sanction  of  his  name. 

It  cannot  be  denied  that  if  spectroscopic  analysis 
had  demonstrated  that  the  elements  in  the  sun  were 
totally  different  from  the  elements  in  the  earth  a  serious 
blow  would  have  been  dealt  to  the  nebular  theory.  The 
collateral  evidence,  strong  as  it  undoubtedly  is,  might 
hardly  have  withstood  so  damaging  an  admission.  If, 
on  the  other  hand,  we  find,  as  we  actually  have 
found,  that  the  elements  in  the  sun  and  the  elements 
in  the  earth  are  practically  identical,  we  obtain  the 
most  striking  corroboration  of  the  truth  of  the  nebular 
theory.  Had  Kant  and  Laplace  been  aware  of  this 
most  significant  fact,  they  would  probably  have  cited 
it  as  most  important  testimony.  They  would  have 
pointed  out  that  the  iron  so  abundant  in  the  earth 
beneath  our  feet  is  also  abundant  in  the  sun  overhead. 
They  would,  I  doubt  not,  if  they  had  known  it,  have 
dwelt  upon  the  circumstance  that  with  that  element, 
carbon,  which  enters  into  every  organic  body  on  this 


THE  NEBULAR  THEORY  STRENGTHENED.  293 

earth,  our  sun  is  also  richly  supplied,  and  they  would 
have  hardly  failed  to  allude  to  the  wide  distribution 
in  space  of  calcium,  hydrogen,  and  many  other  well- 
known  elements. 

Laplace  mainly  based  his  belief  in  the  nebular 
theory  on  some  remarkable  deductions  from  the  theory 
of  probabilities.  To  the  consideration  of  these  we 
proceed  in  the  next  three  chapters.  We  may,  how- 
ever, remark  at  the  outset  that  if  the  evidence  derived 
from  probabilities  seemed  satisfactory  to  Laplace  one 
hundred  years  ago,  this  same  line  of  evidence,  strength- 
ened as  it  has  been  by  recent  discoveries,  is  enor- 
mously more  weighty  at  the  present  day. 


CHAPTER  XIV. 

THE    FIRST    CONCORD. 

Certain  Remarkable  Coincidences — The  Plane  of  Movement  of  a  Planet 
— Consideration  of  Planes  of  Several  Planetary  Orbits— A  Charac- 
teristic of  the  Actual  Planetary  Motions  not  to  be  Explained  by 
Chance— The  First  Concord— The  Planes  not  at  Random— A  Division 
of  the  Right  Angle — Statement  of  the  Coincidences — An  Illustration 
by  Parable  —  The  Cause  of  the  Coincidences  —  The  Argument 
Strengthened  by  the  Asteroids — An  Explanation  by  the  Nebular 
Theory. 

IN  the  present  chapter,  and  in  the  two  chapters 
which  are  to  follow,  I  propose  to  give  an  outline  of 
those  arguments  in  favour  of  the  nebular  theory 
which  are  presented  by  certain  remarkable  coincidences 
observed  in  the  movements  of  the  bodies  of  our  solar 
system.  There  are,  indeed,  certain  features  in  the 
movements  of  the  planets  which  would  seem  so  inex- 
plicable if  the  arrangement  of  the  system  had  taken 
place  by  chance,  that  it  is  impossible  not  to  seek  for 
some  physical  explanation.  We  have  already  had 
occasion  to  refer  in  previous  chapters  to  the  move- 
ments of  the  bodies  of  our  system.  It  will  be  our 
object  at  present  to  show  that  it  is  hardly  conceiv- 
able that  the  movements  could  have  acquired  the 
peculiar  characteristics  they  possess  unless  the  solar 


A   PRELIMINARY   POINT.  295 

system  has  itself  had  an  origin  such  as  that  which 
the  nebular  theory  assigns. 

The  argument  on  which  we  are  to  enter  is,  it  must 
be  confessed,  somewhat  subtle,  but  its  cogency  is  irre- 
sistible. For  this  argument  we  are  indebted  to  one 
of  the  great  founders  of  the  nebular  theory.  It  was 
given  by  Kant  himself  in  his  famous  essay. 

We  will  commence  with  a  preliminary  point  which 
relates  to  elementary  mechanics.  It  may,  however, 
help  to  clear  up  a  difficult  point  in  our  argument  if 
I  now  state  some  well-known  principles  in  a  manner 
specially  adapted  for  our  present  purpose. 

Let  us  think  of  two  bodies,  A  and  S,  and,  for  the 
sake  of  clearness,  we  may  suppose  that  each  of  these 
bodies  is  a  perfect  sphere.  We  might  think  of  them 
as  billiard  balls,  or  balls  of  stone,  or  balls  of  iron. 
We  shall,  however,  suppose  them  to  be  formed  of 
material  which  is  perfectly  rigid.  They  may  be  of 
any  size  whatever,  large  or  small,  equal  or  unequal. 
One  of  them  may  be  no  greater  than  a  grain  of  mustard- 
seed,  and  the  other  may  be  as  large  as  the  moon  or 
the  earth  or  the  sun.  Let  us  further  suppose  that 
there  is  no  other  body  in  the  universe  by  which  the 
mutual  attraction  of  the  two  bodies  we  are  considering 
can  be  interfered  with.  If  these  two  bodies  are 
abandoned  to  their  mutual  attraction,  let  us  now  see 
what  the  laws  of  mechanics  assure  us  must  necessarily 
happen. 

Let  A  and  S  be  simply  released  from  initial 
positions  of  absolute  rest.  In  these  circumstances,  the 
two  points  will  start  off  towards  each  other.  The 
time  that  must  elapse  before  the  two  bodies  collide 
will  depend  upon  circumstances.  The  greater  the 


THE    EAETHS    BEGINNING. 


Fig.  45. — A  SPIRAL  PRESENTED  EDGEWISE  (n.g.c.  4631;  in   Coma  Berenices). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

initial  distance  between  the  two  balls;  their  sizes  being 
the  same,  the  longer  must  be  the  interval  before 
they  come  together.  The  relation  between  the  distance 
separating  the  bodies  and  the  time  that  must  elapse 
before  they  meet  may  be  illustrated  in  this  way. 
Suppose  that  two  balls,  both  starting  from  rest  at  a 
certain  distance,  should  take  a  year  to  come  together 
by  their  mutual  attraction,  then  we  know  that  if  the 
distance  of  the  two  balls  had  been  four  times  as 
great  eight  years  would  have  to  elapse  before  the  two 
balls  collided.  If  the  distances  were  nine  times  as  great 


LAWS    OF   MOVEMENT.  297 

then  twenty-seven  years  would  elapse  before  the  balls 
collided,  and  generally  the  squares  of  the  times  would 
increase  as  the  cubes  of  the  distances.  In  such  state- 
ments we  are  supposing  that  the  radii  of  the  balls  are 
inconsiderable  in  comparison  with  the  distances  apart 
from  which  they  are  started.  The  time  occupied  in  the 
journey  must  also  generally  depend  on  the  masses  of 
the  two  bodies,  or,  to  speak  more  precisely,  on  the  sum 
of  the  masses  of  the  two  bodies.  If  the  two  balls  each 
weighed  five  hundred  tons,  then  they  would  take  pre- 
cisely the  same  time  to  rush  together  as  would  two 
balls  of  one  ton  and  nine  hundred  and  ninety-nine 
tons  respectively,  provided  the  distances  between  the 
centres  of  the  two  balls  had  been  the  same  in  each 
case.  If  the  united  masses  of  the  two  bodies  amounted 
to  four  thousand  tons,  then  they  would  meet  in  half 
the  time  that  would  have  been  required  if  their  united 
masses  were  one  thousand  tons,  it  being  understood 
that  in  each  case  they  started  with  the  same  initial 
distance  between  the  centres. 

Instead  of  simply  releasing  the  two  bodies  A  and 
S  so  that  neither  of  them  shall  have  any  impulse 
tending  to  make  it  swerve  from  the  line  directly  joining 
them,  let  us  now  suppose  that  we  give  one  of  the 
bodies,  A,  a  slight  push  sideways.  The  question  will 
be  somewhat  simpler  if  we  think  of  S  as  very  massive, 
while  A  is  relatively  small.  If,  for  instance,  S  be  as 
heavy  as  a  cannon-ball,  while  A  is  no  heavier  than  a 
grain  of  shot,  then  we  may  consider  that  S  remains 
practically  at  rest  during  the  movement.  The  small 
pull  which  A  is  able  to  give  will  produce  no  more 
than  an  inappreciable  effect  on  S.  If  the  two  bodies 
come  together,  A  will  practically  do  all  the  moving. 


298 


THE   EARTH'S    BEGINNING. 


We  represent  the  movement  in  the  adjoining  figure. 
If  A  is  started  off  with  an  initial  velocity  in  the 
direction  AT,  the  attraction  of  S  will,  however,  make 

itself  felt,  even 
though  A  cannot 
move  directly  to- 
wards S.  The  body 
will  not  be  allowed 
to  travel  along  AT; 
it  will  be  forced  to 
swerve  by  the  attrac- 
tion of  S;  it  will 
move  from  P  to 

Fig.  46.-THB  PLANE  OP  A  PLANET'S  Q>   gradually   getting 

ORBIT.  nearer  to  S.   To  enter 

into    the    details    of 

the  movement  would  require  rather  more  calculation 
than  it  would  be  convenient  to  give  here.  Even  though 
S  is  much  more  massive  than  A,  we  may  suppose  that 
the  path  which  A  follows  is  so  great  that  the  diameter 
of  the  globe  S  is  quite  insignificant  in  comparison  with  the 
diameter  of  the  orbit  which  the  smaller  body  describes. 
We  shall  thus  regard  both  A  and  S  as  particles,  and 
Kepler's  well-known  law,  to  which  we  so  often  refer, 
tells  us  that  A  will  revolve  around  S  in  that  beautiful 
figure  which  the  mathematician  calls  an  ellipse.  For 
our  present  purpose  we  are  particularly  to  observe  that 
the  movement  is  restricted  to  a  plane.  The  plane  in 
which  A  moves  depends  entirely  on  the  direction  in 
which  it  was  first  started.  The  body  will  always  con- 
tinue to  move  in  the  same  plane  as  that  in  which  its 
motion  originally  commenced.  This  plane  is  determined 
by  the  point  S  and  the  straight  line  in  which  A  was 


PLANES    OF    TWO   PLANETARY   ORBITS.       299 

originally  projected.  It  is  essential  for  our  argument 
to  note  that  A  will  never  swerve  from  its  plane  so 
long  as  there  are  not  other  forces  in  action  beside 
those  arising  from  the  mutual  attractions  of  A  and  S. 
The  ordinary  perturbations  of  one  body  by  the  action 
of  others  need  not  here  concern  us. 

The  case  we  have  supposed  will,  of  course,  include 
that  of  the  movement  of  a  planet  round  the  sun. 
The  planet  is  small  and  represented  by  the  body  A, 
which  revolves  round  the  great  body  S,  which  stands 
for  the  sun.  However  the  motion  of  the  planet  may 
actually  have  originated,  it  moves  just  as  if  it  had 
received  a  certain  initial  impulse,  in  consequence  of 
which  it  started  into  motion,  and  thus  defined  a 
certain  plane,  to  which  for  all  time  its  motion  would 
be  restricted. 

So  far  we  have  spoken  of  only  a  single  planet ;  let  us 
now  suppose  that  a  second  planet,  B,  is  also  to  move  in 
revolution  about  the  same  sun.  This  planet  may  be  as 
great  as  A,  or  bigger,  or  smaller,  but  we  shall  still 
assume  that  both  planets  are  inconsiderable  in  com- 
parison with  S.  We  may  assume  that  B  revolves  at 
the  same  distance  as  A,  or  it  may  be  nearer,  or  further. 
The  orbit  of  B  might  also  have  been  in  the  same  plane 
as  A,  or — and  here  is  the  important  point — it  might  have 
been  in  a  plane  inclined  at  any  angle  whatever  to  the 
orbit  of  A.  The  two  planes  might,  indeed,  have  been 
perpendicular.  No  matter  how  varied  may  be  the 
circumstances  of  the  two  planets,  the  sun  would  accept 
the  control  of  each  of  them ;  each  would  be  guided  in 
its  own  orbit,  whether  that  orbit  be  a  circle,  or  whether  it 
be  an  ellipse  of  any  eccentricity  whatever.  So  far  as 
the  attraction  of  the  sun  is  concerned,  each  of  these 


300  THE    EARTH'S    BEGINNING. 

planets  would  remain  for  ever  in  the  same  plane  as 
that  in  which  it  originally  started.  Let  us  now  suppose 
a  third  planet  to  be  added.  Here  again  we  may  assume 
every  variety  in  the  conditions  of  mass  and  distance. 
We  may  also  assume  that  the  plane  which  contains 
the  orbit  of  this  third  planet  is  inclined  at  any  angle 
whatever  to  the  planes  of  the  preceding  planets.  In 
the  same  way  we  may  add  a  fourth  planet,  and  a  fifth ; 
and  in  order  to  parallel  the  actual  circumstance  of 
our  solar  system,  so  far  as  its  more  important  members 
are  concerned,  we  may  add  a  sixth,  and  a  seventh,  and 
an  eighth.  The  planes  of  these  orbits  are  subjected 
to  a  single  condition  only.  Each  one  of  them  passes 
through  the  centre  of  the  sun.  If  this  requirement  is 
fulfilled,  the  planes  may  be  in  other  respects  as  different 
as  possible. 

In  the  actual  solar  system  the  circumstances  are, 
however,  very  different  from  what  we  have  represented 
in  this  imaginary  solar  system.  It  is  the  most  obvious 
characteristic  of  the  tracks  of  Jupiter  and  Venus,  and 
the  other  planets  belonging  to  the  sun,  that  the 
planes  in  which  they  respectively  move  coincide  very 
nearly  with  the  plane  in  which  the  earth  revolves. 
We  must  suppose  all  the  orbits  of  our  imaginary 
system  to  be  flattened  down,  nearly  into  a  plane,  before 
we  can  transform  the  imaginary  system  of  planets  I 
have  described  into  the  semblance  of  an  actual  solar 
system. 

If  the  orbits  of  the  planets  had  been  arranged 
in  planes  which  were  placed  at  random,  we  may 
presume  they  would  have  been  inclined  at  very  varied 
angles.  As  they  are  not  so  disposed,  we  may  conclude 
that  the  planes  have  not  been  put  down  at  random; 


WHAT   IS    THE    REASON? 


301 


we  must  conclude  that  there  has  been  some  cause  in 
action  which,  if  we  may  so  describe  it,  has  superintended 
the  planes  of  these  orbits  and  ordained  that  they  should 
be  placed  in  a  very  particular  manner. 

Two  planets'  orbits  might  conceivably  coincide  or 
be  perpendi- 
cular, or  they 
might  con- 
tain any  in- 
termediate 
angle.  The 
plane  of  the 
second  planet 
might  be  in- 
clined to  the 
first  at  an 
angle  con- 
taining any 
number  of 
degrees.  To 
make  some 
numerical 
estimate  of 

the  matter,  we  proceed  as  follows  :  If  we  divide  the  right 
angle  into  ten  parts  of  nine  degrees  each  (Fig.  47),  then 
the  inclination  of  the  two  planes  might,  for  example,  lie 
between  0°  and  9°,  or  between  18°  and  27°,  or  between 
45°  and  54°,  or  between  81°  and  90°,  or  in  any  one 
of  the  ten  divisions.  Let  us  think  of  the  oruit  of  Jupiter. 
Then  the  inclination  of  the  plane  in  which  it  moves  to 
the  plane  in  which  the  earth  moves  must  fall  into  one 
of  the  ten  divisions.  As  a  matter  of  fact,  it  does  fall 
into  the  angle  between  0°  and  9°. 


Fig.  47. — A  RIGHT  ANGLE  DIVIDED  INTO  TEN  PARTS. 


302  THE    EARTH'S    BEGINNING. 

But  now  let  us  consider  a  second  planet,  for  example 
Venus.  If  the  orbit  of  Venus  were  to  be  placed  at 
random,  its  inclination  might  with  equal  probability  lie 
in  any  one  of  the  ten  divisions,  each  of  nine  degrees,  into 
which  we  have  divided  the  right  angle.  It  would  be 
just  as  likely  to  lie  between  forty-five  and  fifty-four, 
or  between  seventy-two  and  eighty-one,  as  in  any 
other  division.  But  we  find  another  curious  coincidence. 
It  was  already  remarkable  that  the  plane  of  Jupiter's 
orbit  should  have  been  included  in  the  first  angle  of 
nine  degrees  from  the  orbit  of  the  earth.  It  is  therefore 
specially  noteworthy  to  find  that  the  planet  Venus 
follows  the  same  law,  though  each  one  of  the  ten  angular 
divisions  was  equally  available. 

The  coincidences  we  have  mentioned,  remarkable 
as  they  are,  represent  only  the  first  of  the  series.  What 
has  been  said  with  respect  to  the  positions  of  the 
orbits  of  Jupiter  and  Venus  may  be  repeated  with 
regard  to  the  orbits  of  Mercury  and  Mars,  Saturn, 
Uranus,  and  Neptune.  If  the  tracks  of  these  planets 
had  been  placed  merely  at  random,  their  inclina- 
tions would  have  been  equally  likely  to  fall  into  any 
of  the  ten  divisions.  As  a  matter  of  fact,  they  all 
agree  in  choosing  that  one  particular  division  which 
is  adjacent  to  the  track  of  the  earth.  If  the  orbits  of 
the  planets  had  indeed  been  arranged  fortuitously,  it 
is  almost  inconceivable  that  such  coincidences  could 
have  occurred.  Let  me  illustrate  the  matter  by  the 
following  little  parable. 

There  were  seven  classes  in  a  school,  and  there 
were  ten  boys  in  each  class.  There  was  one  boy 
named  Smith  in  the  first  class,  but  only  one.  There 
was  also  one  Smith,  but  only  one,  in  each  of  the 


THE   "SMITH"    PARABLE.  303 

other  classes.  The  others  were  named  Brown,  Jones, 
Robinson,  etc.  An  old  boy,  named  Captain  Smith, 
who  had  gone  out  to  Australia  many  years  before, 
came  back  to  visit  his  old  school.  He  had  succeeded 
well  in  the  world,  and  he  wanted  to  do  something 
generous  for  the  boys  at  the  place  of  which  he  had 
such  kindly  recollections.  He  determined  to  give  a 
plum-cake  to  one  boy  in  each  class ;  and  the  fortunate 
boy  was  to  be  chosen  by  lot.  The  ten  boys  in  each 
class  were  to  draw,  and  each  successful  boy  was  to 
be  sent  in  to  Captain  Smith  to  receive  his  cake. 

The  Captain  sat  at  a  table,  and  the  seven  winners 
were  shown  in  to  receive  their  prizes.  "  What  is  your 
name  ? "  he  said  to  the  boy  in  the  first  class,  as  he 
shook  hands  with  him.  "  Smith,"  replied  the  boy. 
"  Dear  me,"  said  the  Captain,  "  how  odd  that  our  names 
should  be  the  same.  Never  mind,  it's  a  good  name. 
Here's  your  cake.  Good-bye,  Smith."  Then  up 
came  the  boy  from  the  second  class.  "  What  is  your 
name  ?  "  said  the  Captain.  "  Smith,  sir,"  was  the  reply. 
"  Dear  me,"  said  the  visitor.  "  This  is  very  singular. 
It  is  indeed  a  very  curious  coincidence  that  two  Smiths 
should  have  succeeded.  Were  you  really  chosen  by 
drawing  lots  ?  "  "  Yes,  sir,"  said  the  boy.  "  Then  are 
all  the  boys  in  your  class  named  Smith  ? "  "  No,  sir ; 
I'm  the  only  one  of  that  name  in  the  ten."  "Well," 
said  the  Captain,  "  it  really  is  most  curious.  I  never 
heard  anything  so  extraordinary  as  that  two  name- 
sakes of  my  own  should  happen  to  be  the  winners. 
Now  then  for  the  boy  from  class  three."  A  cheerful 
youth  advanced  with  a  smile.  "  Well,  at  all  events,'* 
said  the  good-natured  old  boy,  "your  name  is  not 
Smith?"  "Oh,  but  it  is,"  said  the  youth.  The 


304  THE   EARTH'S   BEGINNING. 

gallant  Captain  jumped  up,  and  declared  that  there 
must  have  been  some  tremendous  imposition.  Either 
the  whole  school  consisted  of  Smiths,  or  they  called 
themselves  Smiths,  or  they  had  picked  out  the 
Smiths.  The  four  remaining  boys,  still  expecting 
their  cakes,  here  burst  out  laughing.  "  What  are  your 
names?"  shouted  the  donor.  "Smith!"  "Smith!!" 
"  Smith  ! ! !  "  "  Smith  MIL"  were  the  astounding  replies. 
The  good  man  could  stand  this  no  longer.  He  sent 
for  the  schoolmaster,  and  said,  "  I  particularly  requested 
that  you  would  choose  a  boy  drawn  by  lot  from  each 
of  your  seven  classes,  but  you  have  not  done  so.  You 
have  merely  picked  out  my  namesakes  and  sent  them 
up  for  the  cakes."  But  the  master  replied,  "  No,  I 
assure  you,  they  have  been  honestly  chosen  by  lot. 
Nine  black  beans  and  one  white  bean  were  placed  in 
a  bag ;  each  class  of  ten  then  drew  in  succession, 
and  in  each  class  it  happened  that  the  boy  named 
Smith  drew  the  white  bean." 

"  But,"  said  the  visitor,  "  this  is  not  credible.  Only 
once  in  ten  million  times  would  all  the  seven  Smiths 
have  drawn  the  white  beans  if  left  solely  to  chance. 
And  do  you  mean  to  tell  me  that  what  can  happen 
only  once  out  of  ten  million  times  did  actually  happen 
on  this  occasion — the  only  occasion  in  my  life  on  which 
I  have  attempted  such  a  thing  ?  I  don't  believe  the 
drawing  was  made  fairly  by  lot.  There  must  have 
been  some  interference  with  the  operation  of  chance. 
I  insist  on  having  the  lots  drawn  again  under  my 
own  inspection."  "  Yes,  yes,"  shouted  all  the  other 
boys.  But  all  the  successful  Smiths  roared  out,  "  No." 
They  did  not  feel  at  all  desirous  of  another  trial.  They 
knew  enough  of  the  theory  of  probabilities  to  be  aware 


A    SATISFACTORY  ENDING.  305 

that  they  might  wait  till  another  ten  million  fortunate 
old  boys  came  back  to  the  school  before  they  would 
have  such  luck  again.  The  situation  carrie  to  a  dead- 
lock. The  Captain  protested  that  some  fraud  had 
been  perpetrated,  and  in  spite  of  their  assurances  he 
would  not  believe  them.  The  seven  Smiths  declared 
they  had  won  their  cakes  honestly,  and  that  they  would 
not  surrender  them.  The  Captain  was  getting  furious, 
the  boys  were  on  the  point  of  rebellion,  when  the 
schoolmaster's  wife,  alarmed  by  the  tumult,  came  on 
the  scene.  She  asked  what  was  the  cause  of  the  dis- 
turbance. It  was  explained  to  her,  and  then  Captain 
Smith  added  that  by  mathematical  probabilities  it  was 
almost  inconceivable  that  the  only  seven  Smiths  in 
the  school  should  have  been  chosen.  The  gracious 
lady  replied  that  she  knew  nothing,  and  cared  as  little, 
about  the  theory  of  probabilities,  but  she  did  care 
greatly  that  the  school  should  not  be  thrown  into 
tumult.  "  There  is  only  one  solution  of  this  difficulty," 
she  added.  "It  is  that  you  forthwith  provide  cakes, 
not  only  for  the  seven  Smiths,  but  for  every  one  of 
the  boys  in  the  school."  This  resolute  pronouncement 
was  received  with  shouts  of  approval.  The  Captain, 
with  a  somewhat  rueful  countenance,  acknowledged 
that  it  only  remained  for  him  to  comply.  He  returned, 
shortly  afterwards,  to  his  gold-diggings  in  Australia, 
there  to  meditate  during  his  leisure  on  this  remarkable 
illustration  of  the*  theory  of  probabilities. 

This  parable  illustrates  the  improbability  ol  such 
arrangements  as  we  find  in  the  planets  having 
originated  by  chance.  The  chances  against  their 
having  thus  occurred  are  10,000,000  to  1.  Hence  we 
find  it  reasonable  to  come  to  the  conclusion  that  the 


306  THE   EARTH'S    BEGINNING. 

arrangement,  by  which  the  planets  move  round  the 
sun  in  planes  which  are  nearly  coincident,  cannot  have 
originated  by  chance.  There  must  have  been  some  cause 
which  produced  this  special  disposition.  We  have, 
therefore,  to  search  for  some  common  cause  which 
must  have  operated  on  all  the  planets.  As  the  planets 
are  at  present  absolutely  separated  from  each  other, 
it  is  impossible  for  us  to  conceive  a  common  cause  acting 
upon  them  in  their  present  condition.  The  cause  must 
have  operated  at  some  primaeval  time,  before  the  planets 
assumed  the  separate  individual  existence  that  they 
now  have. 

We  have  spoken  so  far  of  the  great  planets  only, 
and  we  have  seen  how  the  probability  stands.  We 
should  also  remark  that  there  are  also  nearly  500  small 
planets,  or  asteroids,  as  they  are  more  generally  called. 
Among  them  are,  no  doubt,  a  few  whose  orbits  have 
inclinations  to  the  ecliptic  larger  than  those  of  the 
great  planets.  The  great  majority  of  the  asteroids 
revolve,  however,  very  close  to  that  remarkable  plane 
with  which  the  orbits  of  the  great  planets  so  nearly 
coincide.  Every  one  of  these  asteroids  increases  the 
improbability  that  the  planes  of  the  orbits  could  have 
been  arranged  as  we  find  them,  without  some  special 
disposing  cause.  It  is  not  necessary  to  write  down 
an  immense  string  of  figures.  The  probability  is 
absolutely  overwhelming  against  such  an  arrangement 
being  found  if  the  orbits  of  the  planets  had  been  decided 
by  chance,  and  chance  alone. 

We  may  feel  confident  that  there  must  have  been 
some  particular  circumstances  accompanying  the  form- 
ation of  the  solar  system  which  rendered  it  absolutely 
necessary  for  the  orbits  of  the  planets  to  possess  this 


THE    NEBULAR    THEORY  AGAIN.  307 

particular  characteristic.  We  have  pointed  out  in 
Chapter  XII.  that  the  nebular  theory  offers  such  an 
explanation,  and  we  do  not  know  of  any  other  natural 
explanation  which  would  be  worthy  of  serious  attention. 
Indeed,  we  may  say  that  no  other  such  explanation 
has  ever  been  offered. 


CHAPTER     XV. 

THE     SECOND     CONCORD. 

Another  Remarkable  Coincidence  in  the  Solar  System— The  Second 
Concord — The  Direction  of  the  Movements  of  the  Great  Planets— 
The  Movement  of  Ceres — Yet  Another  Planet — Discovery  of  Eros — 
The  Neai^est  Neighbour  of  the  Earth — Throwing  Heads  and  Tails — 
A  Calculation  of  the  Chances— The  Numerical  Strength  of  the 
Argument — An  Illustration  of  the  Probability  of  the  Origin  of  the 
Solar  System  from  the  Nebula— The  Explanation  of  the  Second 
Concord  offered  by  the  Nebular  Theory — The  Relation  of  Energy 
and  Moment  of  Momentum — Different  Systems  Ilhistrated — That  all 
the  Movements  should  be  in  the  same  Direction  is  a  Consequence  of 
Evolution  from  the  Primaeval  Nebula. 

WE  have  seen  in  the  last  chapter  that  there  is  a  very 
remarkable  concordance  in  the  positions  of  the  planes 
of  the  orbits  of  the  planets,  and  we  have  shown  that 
this  concordance  finds  a  natural  historical  explanation 
in  the  nebular  origin  of  our  system.  We  have  now  to 
consider  another  striking  concord  in  the  movements 
of  the  planets  in  their  several  orbits,  and  this  also 
furnishes  us  with  important  evidence  as  to  the  truth 
of  the  nebular  theory.  The  argument  on  which  we  are 
now  to  enter  is  one  which  specially  appealed  to  Laplace, 
and  was  put  forward  by  him  as  the  main  foundation  of 
the  nebular  theory. 


THE    SOLAR    SYSTEM. 


309 


.  48.  —  ILLUSTRATION  OF  THE  SECOND  CONCORD. 


In  the  adjoining  Fig.  48  we  have  a  diagram  of  a 
portion  of  the  solar  system.  We  shall  regard  the  move- 
ments as  somewhat  simplified.  The  sun  is  supposed 
to  be  at  the  centre,  turning  round  once  every  twenty-five 
days,  on  an  axis  which  is  supposed  to  be  perpendicular 
to  the  plane  of  the  paper.  We  may  also  for  our 
present  purpose  assume  that  the  orbits  of  the  earth 
and  the  other  planets  lie  in  this  same  plane. 

In    the    first    place    we    observe    that    the    earth 
21 


310  TH$   EARTH'S   BEGINNING. 

might  have  gone  round  its  track  in  either  direction 
so  far  as  the  welfare  of  mankind  is  concerned.  The 
succession  of  day  and  night,  and  the  due  changes  of 
the  seasons,  could  have  been  equally  well  secured 
whichever  be  the  direction  in  which  the  earth  revolves. 
We  do,  however,  most  certainly  find  that  the  direction 
in  which  the  earth  revolves  round  the  sun  is  the  same 
as  the  direction  in  which  the  sun  rotates  on  its  axis. 
This  is  the  first  coincidence. 

We  may  now  consider  other  planets.  Look,  for 
instance,  at  the  orbit  of  Jupiter.  It  seems  obvious 
that  Jupiter  might  have  been  made  to  revolve  round 
the  sun  either  one  way  or  the  other;  indeed,  it  will 
be  remembered  that  though  Kepler's  laws  indicate 
so  particularly  the  shape  of  the  track  in  which  the 
planet  revolves,  and  prescribe  so  beautifully  the  way 
in  which  the  planet  must  moderate  or  accelerate  its 
velocity  at  the  different  parts  of  its  track,  yet  they  are 
quite  silent  as  to  the  direction  in  which  the  planets 
shall  revolve  in  that  track.  If  we  could  imagine  a 
planet  to  be  stopped  to  have  its  velocity  reversed,  and 
then  to  be  started  in  a  precisely  opposite  direction,  it 
would  still  continue  to  describe  precisely  the  same 
path;  it  would  still  obey  Kepler's  laws  with  unfailing 
accuracy,  so  far  as  our  present  argument  is  concerned, 
and  the  velocity  which  it  would  have  at  each  point  ^ 
of  the  track  would  be  quite  the  same  whether  the  planet 
were  going  one  way  or  whether  it  was  going  the  other. 
It  is  therefore  equally  possible  for  Jupiter  to  pursue 
his  actual  track  by  going  round  the  sun  in  the  same 
direction  as  the  earth,  or  by  going  in  the  opposite 
direction.  But  we  actually  find  that  Jupiter  does 
take  the  same  direction  as  the  earth,  and  this,  as  we 


COINCIDENCES.  311 

have  already  seen,  is  the  direction  in  which  the  sun 
rotates.  Here  we  have  the  second  coincidence. 

We  now  take  another  planet;  for  example,  Mars. 
Again  we  affirm  that  Mars  could  have  moved  in  either 
direction,  but,  as  a  matter  of  fact,  it  pursues  the  same 
direction  as  Jupiter  and  the  earth.  In  the  orbital 
movement  of  Saturn  we  have  the  fourth  coincidence 
of  the  same  kind,  and  we  have  a  fifth  in  the  case  of 
Mercury,  and  a  sixth  in  Venus,  a  seventh  in  Uranus, 
and  an  eighth  in  Neptune.  The  seven  great  planets 
and  the  earth  all  revolve  around  the  sun,  not  only  in 
orbits  which  are  very  nearly  in  the  same  plane,  but  they 
also  revolve  in  the  same  direction. 

The  coincidences  we  have  pointed  out  with  regard 
to  the  movements  of  the  great  planets  of  our  system 
may  be  also  observed  with  regard  to  the  numerous 
bodies  of  asteroids.  On  the  first  night  of  the  century 
just  closed,  the  1st  of  January,  1801,  the  first  of  the 
asteroids,  now  known  as  Ceres,  was  discovered.  This 
was  a  small  planet,  not  a  thousandth  part  of  the  bulk 
of  one  of  the  older  planets,  and  visible,  of  course,  only  in 
the  telescope.  Like  the  older  planets,  it  was  found  to 
obey  Kepler's  laws  ;  but  this  we  might  have  foreseen, 
because  Kepler's  laws  depend  upon  the  attraction  of 
gravitation,  and  must  apply  to  any  planet,  whatever 
its  size.  When,  therefore,  the  new  planet  was  found, 
and  its  track  was  known,  it  was  of  much  interest  to 
see  whether  the  planet  in  moving  round  that  track 
observed  the  same  direction  in  which  all  the  older 
planets  had  agreed  to  travel,  or  whether  it  moved  in 
the  opposite  direction.  In  the  orbit  of  Ceres  we  have 
a  repetition  of  the  coincidence  which  has  been  noticed  in 
each  of  the  other  planets.  The  new  planets,  like  all 


312  THE    EARTH'S    BEGINNING. 

the  rest,  move  round  the  sun  in  the  same  direction 
as  the  sun  rotates  on  its  axis.  The  discovery  of  this 
first  asteroid  was  quickly  followed  by  other  similar  dis- 
coveries; each  of  the  new  planets  described,  of  course, 
an  ellipse,  and  the  directions  which  these  planets  fol- 
lowed in  their  movements  round  the  sun  were  in 
absolute  harmony  with  those  of  the  older  planets. 

But,  besides  the  great  planets  and  the  asteroids 
properly  so  called,  there  is  yet  another  planet,  Eros.  Its 
testimony  is  of  special  value,  inasmuch  as  it  seems  to 
stand  apart  from  all  other  bodies  in  the  solar  system, 
and  with,  of  course,  the  exception  of  the  moon,  it  is 
the  earth's  nearest  neighbour.  But  whatever  may  be  the 
exceptional  features  of  Eros,  however  it  may  differ  from 
the  great  planets  and  the  asteroids  already  known,  yet 
Eros  makes  no  exception  to  the  law  which  we  have 
found  to  be  obeyed  by  all  the  other  planets.  It  also 
revolves  round  the  sun  in  the  same  direction  as  all  the 
planets  revolve,  in  the  same  direction  as  the  rotation  of 
the  sun  (Fig.  49). 

We  may  pause  at  this  moment  to  make  a  calcula- 
tion as  to  the  improbability  that  the  sun,  the  earth,  the 
seven  great  planets,  and  Ceres,  numbering  altogether 
ten,  should  move  round  in  the  same  direction  if  their 
movements  had  been  left  to  chance.  This  will  show 
what  we  can  reasonably  infer  from  this  concord  in 
their  movements.  The  theory  of  probabilities  will 
again  enlighten  a  difficult  subject. 

There  are  only  two  possible  directions  for  the  motion 
of  a  planet  in  its  orbit.  It  must  move  like  the  hands 
of  a  watch,  or  it  must  move  in  the  opposite  direction, 
The  planet  must  move  one  way  or  the  other,  just 
as  a  penny  must  always  fall  head  or  tail 


HEADS    OR    TAILS?  313 

We  may  illustrate  this  remarkable  coincidence  in 
the  following  manner:  Suppose  we  take  ten  coins  in 
the  hand,  and  toss  them  all  up  together  and  let  them 
fall  on  the  table  ;  in  the  vast  majority  of  cases  in 
which  the  experiment  may  be  tried,  there  would  be 
some  heads  and  some  tails ;  they  would  not  all  be  heads. 


Fig.  49.— ORBITS  OF  EARTH,  EROS  AND  MAUS. 

But  it  is,  of  course,  not  impossible  that  the  coins  should 
all  turn  up  heads.  We  should,  however,  deem  it  a  very 
remarkable  circumstance  if  it  happened :  yet  it  would 
certainly  not  be  more  remarkable  than  that  the  ten 
celestial  movements  should  all  take  place  in  the  same 
direction,  unless,  indeed,  it  should  turn  out  that  there 
is  some  sound  physical  cause  which  imposes  on  the 
planets  of  the  solar  system  an  obligation,  restricting 


314  THE   EARTH'S    BEGINNING. 

their  movements  round  the  sun  to  the  same  direction 
as  that  in  which  the  sun  itself  rotates. 

It  will  be  useful  to  study  the  matter  numerically; 
and  the  rules  of  probabilities  will  enable  us  to  do  so,  as 
we  may  see  by  the  following  illustration  :  We  deem 
the  captain  of  a  cricket  team  fortunate  when  he  wins 
the  toss  for  innings.  We  should  deem  him  lucky 
indeed  if  he  won  it  three  times  in  successive  matches. 
If  he  won  it  five  times  running,  his  luck  would  be 
phenomenal ;  while,  if  it  was  stated  that  he  won  it  ten 
times  consecutively,  we  should  consider  the  statement 
well-nigh  incredible.  For  it  is  easy  to  calculate  that  the 
chances  against  such  an  occurrence  are  one  thousand 
and  twenty-four  to  one.  In  like  manner  we  may  say, 
that  for  nine  planets  and  the  sun  all  to  go  round  in 
the  same  direction  would  be  indeed  surprising  if  the 
arrangement  of  the  planets  had  been  determined  by 
chance;  there  are  more  than  a  thousand  chances  to  one 
against  such  an  occurrence. 

But  Ceres  was  only  the  earliest  of  many  other 
similar  discoveries.  And  as  each  asteroid  was  success- 
ively brought  to  light,  it  became  most  interesting  to 
test  whether  it  followed  the  rest  of  the  planets  in  that 
wonderful  unanimity  in  the  direction  of  their  movements 
of  revolution,  or  whether  it  made  a  new  departure  by 
going  in  the  opposite  direction.  No  such  exception  has 
ever  yet  been  observed.  Let  us  take,  then,  ten  more 
planets,  in  addition  to  those  we  have  already  considered, 
so  that  we  have  now  nineteen  planets  all  revolving  in 
the  same  direction  as  the  sun  rotates.  It  is  easy  to 
compute  the  improbability  that  these  twenty  move- 
ments should  all  be  in  the  same  direction,  if,  indeed,  it 
were  by  chance  that  their  directions  had  been  determined. 


CALCULATING    CHANCES.  315 

It  is  the  same  problem  as  the  following :  What  is  the 
chance  that  twenty  coins,  taken  together  in  the  hand 
and  tossed  into  the  air  at  once,  shall  all  alight  with 
their  heads  uppermost  ?  We  have  seen  that  the  chances 
against  this  occurrence,  if  there  were  ten  coins,  is  about 
a  thousand  to  one.  It  can  easily  be  shown  that  if  there 
were  twenty  coins  the  chances  against  the  occurrence 
would  be  a  million  to  one.  We  thus  see  that,  even 
with  no  more  than  nineteen  planets  and  the  sun,  there 
is  a  million  to  one  against  a  unanimity  in  the  directions 
of  the  movements,  if  the  determination  of  the  motions 
was  made  by  chance.  We  may,  however,  express  the 
result  in  a  different  manner,  which  is  more  to  the  pur- 
pose of  our  argument.  There  are  a  million  chances  to 
one  in  favour  of  the  supposition  that  the  disposition  of 
the  movements  of  the  planets  has  not  been  the  result  of 
chance ;  or  we  may  say  that  there  are  a  million  chances 
to  one  in  favour  of  the  supposition  that  some  physical 
agent  has  caused  the  unanimity. 

We  can  add  almost  any  desired  amount  of  numerical 
strength  to  the  argument.  The  discoveries  of  minor 
planets  went  on  with  ever-increasing  success  through 
the  whole  of  the  last  century.  When  ten  more  had 
been  found,  and  when  each  one  was  shown  to  obey  the 
same  invisible  guide  as  to  the  direction  in  which  it 
should  pursue  its  elliptic  orbit,  the  chances  in  favour 
of  some  physical  cause  for  the  unanimity  became 
multiplied  by  yet  another  thousand.  The  probability 
then  stood  at  a  thousand  millions  to  one.  As  the  years 
rolled  by,  asteroids  were  found  in  ever-increasing  abund- 
ance. Sometimes  a  single  astronomer  discovered  two, 
and  sometimes  even  more  than  two,  on  a  single  night. 
In  the  course  of  a  lifetime  a  diligent  astronomer 


316  THE   EARTH'S    BEGINNING. 

has  placed  fifty  discoveries  of  asteroids,  or  even  more 
than  fifty,  on  his  record.  By  combined  efforts  the 
tale  of  the  asteroids  has  now  approached  five  hundred, 
and  out  of  that  huge  number  of  independent  planetary 
bodies  there  is  not  one  single  dissentient  in  the  direction 
of  its  motion.  Without  any  exception  whatever,  they 
all  perform  their  revolutions  in  the  same  direction  as 
the  sun  rotates  at  the  centre.  When  this  great  host 
is  considered,  the  numerical  strength  of  the  argument 
has  attained  a  magnitude  too  great  for  expression. 
Each  new  asteroid  simply  doubled  the  strength  of  the 
argument  as  it  stood  before. 

Professor  J.  J.  Thomson  recently  discovered  that 
there  are  corpuscles  of  matter  very  much  smaller  than 
atoms.  Let  us  think  of  one  of  these  corpuscles,  of 
which  many  millions  would  be  required  to  make  the 
smallest  grain  of  sand  which  would  just  be  visible  under 
a  microscope.  Think,  on  the  other  hand,  of  a  sphere 
extending  through  space  to  so  vast  a  distance  that  every 
star  in  the  Milky  Way  will  be  contained  within  its 
compass.  Then  the  number  of  those  corpuscles  which 
would  be  required  to  fill  that  sphere  is  still  far  too  small 
to  represent  the  hugeness  of  the  improbability  that 
all  the  five  hundred  planetary  bodies  should  revolve 
in  the  same  direction,  if  chance,  and  chance  alone, 
had  guided  the  direction  which  each  planet  was  to 
pursue  in  moving  round  its  orbit. 

The  mere  statement  of  these  facts  is  sufficient  to 
show  that  some  physical  agent  must  have  caused  this 
marvellous  concord  in  the  movements  of  the  solar 
system.  How  the  argument  would  have  stood  if  there 
had  been  even  a  single  dissentient  it  is  not  necessary 
to  consider,  for  there  is  no  dissentient.  No  reasonable 


ONCE    MORE    THE   NEBULAR    THEORY.        317 

person  will  deny  that  these  facts  impose  an  obligation  to 
search  for  the  physical  explanation  of  this  feature  in 
the  planetary  movements. 

As  in  the  last  chapter,  where  we  were  dealing  with 
the  positions  of  the  planes  of  the  orbits,  there  can  here  be 
no  hesitation  as  to  the  true  cause  of  this  most  striking 
characteristic  of  the  planetary  movements.  The  nebular 
theory  is  at  once  ready  with  an  explanation,  as  has 
been  already  indicated  in  Chapter  XL  The  primaeval 
nebula,  endowed  in  the  beginning  with  a  certain  amount 
of  moment  of  momentum,  has  been  gradually  con- 
tracting. It  has  been  gradually  expending  its  energy, 
as  we  have  already  had  occasion  to  explain ;  but  the 
moment  of  momentum  has  remained  undiminished 
And  from  this  it  can  be  shown  that  the  dynamical 
principles  guiding  the  evolution  of  the  nebula  must 
ultimately  refuse  permission  for  any  planet  to  revolve 
in  opposition  to  the  general  movement.  This  point  is 
a  very  interesting  one,  and  as  it  is  of  very  great  im- 
portance in  connection  with  our  system,  I  must  give 
it  some  further  illustration  and  explanation. 

The  two  figures  that  are  shown  in  Fig.  50  represent 
two  imaginary  systems.  We  have  a  sun  in  each,  and  we 
have  two  planets  in  each.  The  sun  is  marked  with  the 
letter  S,  and  the  two  planets  are  designated  by  A  and  B. 
For  simplicity  I  have  represented  the  orbits  as  circles, 
and  for  the  same  reason  I  have  left  out  the  rest  of  the 
planets ;  we  shall  also  suppose  the  orbits  of  the  two 
planets  that  are  involved  to  lie  exactly  in  the  same 
plane.  In  the  two  systems  that  I  have  here  supposed,  the 
two  suns  are  to  be  of  the  same  weight,  the  planet  A  in 
one  system  is  of  equal  mass  to  the  planet  A  in  the  other ; 
and  the  planets  B  in  the  two  systems  are  also  equal. 


318  THE    EARTH'S    BEGINNING. 

It  is  also  assumed  that  the  orbit  of  A  in  one  diagram 
shall  be  the  same  as  the  orbit  of  A  in  the  other,  and  that 
the  orbit  of  B  in  one  shall  be  precisely  the  same  as  the 
orbit  of  B  in  the  other.  The  sun  rotates  in  precisely  the 
same  manner  in  both,  and  takes  the  same  time  for  each 
rotation.  A,  in  one  system,  goes  round  in  the  same 


Fig.  50. —  I.  A  NATURAL  SYSTEM  ON  THE  LEFT. 

II.  AN  UNNATURAL  SYSTEM  ON  THE  RIGHT. 

time  that  A  does  in  the  other;  and  B,in  one  system,  goes 
round  in  the  same  time  that  B  does  in  the  other.  There 
is,  therefore,  a  perfect  resemblance  between  the  two 
systems  I  have  here  supposed  in  every  point  but  one. 
I  have  indicated,  as  usual,  the  movements  of  the  bodies 
by  arrows,  and,  while  in  one  of  the  systems  the  sun  and 
A  and  B  all  go  round  in  the  same  direction,  in  the  other 
system  the  sun  and  A  go  round,  no  doubt,  in  the  same 
direction,  but  the  direction  of  B  is  opposite.  We  are 
not,  in  this  illustration,  considering  the  rotations  of 
the  planets  on  their  axes.  That  will  be  dealt  with  in 
the  next  chapter. 

There  can  be  no  doubt  that  either  of  these  two 


ROTATION   OF   PLANETS.  319 

systems  would  be  possible  for  thousands  of  revolutions. 
There  is  nothing  whatever  to  prevent  A  and  B  from 
being  started  in  the  same  direction  round  the  sun  as  in 
the  first  figure,  or  with  A  in  one  direction  and  B  in 
the  opposite  direction,  as  in  the  second  figure.  It  is 
equally  conceivable  that,  while  A  and  B  revolve  in  the 
same  direction,  both  should  be  opposite  to  that  of  the 
sun.  But  one  system  is  permanent,  and  the  other  is  not. 
For,  as  a  matter  of  fact,  we  do  not  find  in  Nature  such 
an  arrangement  as  that  in  the  second  figure,  or  as  that 
in  which  both  the  planets  revolve  in  opposite  directions 
to  the  sun's  rotation ;  what  we  do  find  is,  that  the 
planets  go  round  in  the  same  direction  as  the  sun. 
And  the  explanation  is  undoubtedly  connected  with  the 
important  principle  already  illustrated,  namely,  that 
natural  systems  are  in  a  condition  in  which  the  total 
quantity  of  energy  undergoes  continuous  reduction  in 
comparison  with  the  moment  of  momentum. 

In  the  arrangements  made  in  the  two  figures,  it 
will  be  recollected  that  the  masses  of  the  three  bodies 
were  respectively  the  same,  and  also  their  distances 
apart,  and  their  velocities.  As  the  energy  depends  only 
on  the  masses,  the  distances,  and  the  velocities,  the 
energies  of  the  two  systems  must  be  identical.  But  the 
moment  of  momentum  of  the  two  systems  is  very 
different,  for  while  in  the  one  case  the  sum  of  the 
moments  of  momentum  of  the  sun's  rotation  and  that  of 
the  planet  A,  which  is  going  in  the  same  direction,  are  to 
be  increased  by  the  moment  of  momentum  of  B,  the 
same  is  not  the  case  in  the  other  system.  The  moment  of 
momentum  of  the  sun  and  of  A  conspire,  no  doubt,  and 
must  be  added  together ;  but  as  B  is  revolving  in  the 
opposite  direction,  the  moment  of  momentum  of  this 


320  THE   EARTH'S    BEGINNING. 

planet  has  to  be  subtracted  before  we  obtain  the  nett 
moment  of  momentum  of  the  system.  Hence,  we 
perceive  a  remarkable  difference  between  the  two 
systems;  for,  though  in  each  the  total  energy  is  the 
same,  yet  in  the  latter  case  the  moment  of  momentum 
is  smaller  than  in  the  former. 

It  has  been  pointed  out  that  the  effect  of  the  mutual 
actions  of  the  different  bodies  of  a  system  is  to  lessen,  in 
course  of  time,  the  total  quantity  of  energy  that  they 
receive  in  the  beginning,  while  it  is  not  in  the  power 
of  the  mutual  actions  of  the  particles  of  the  system 
to  affect  the  sum  total  of  the  moment  of  momentum. 
Hence  we  see  that,  so  long  as  the  system  is  isolated 
from  external  interference,  the  tendency  must  ever 
be  towards  the  reduction  of  the  quantity  of  energy 
to  as  low  a  point  as  may  be  compatible  with  the 
preservation  of  the  necessary  amount  of  moment 
of  momentum.  The  first  of  the  two  systems  given 
in  Fig.  50  is  much  more  in  conformity  with  this 
principle  than  the  second.  The  moment  of  momentum 
in  the  former  case  must  be  nearly  as  large  as  could 
be  obtained  by  any  other  disposition  of  the  matter 
forming  it,  with  the  same  amount  of  energy.  But  in 
the  second  diagram  the  moment  of  momentum  is  much 
less,  though  the  energy  is  the  same.  It  follows  that 
the  energy  of  this  system  might  be  largely  reduced, 
for  if  accompanied  by  a  suitable  re-arrangement  of 
the  planets  the  reduced  amount  of  moment  of  momen- 
tum might  be  easily  provided  for.  We  thus  see  that 
this  system  is  not  one  to  which  the  evolution  of  a 
material  arrangement  would  ultimately  tend.  It  is, 
therefore,  not  to  be  expected  in  Nature,  and  we  do 
not  find  it.  Of  course,  the  same  would  be  equally  true 


THE    SECOND    CONCORD.  321 

if,  instead  of  having  merely  two  planets,  as  I  have 
here  supposed  for  the  sake  of  illustration,  the  planets 
were  much  more  numerous.  The  operation  of  the 
causes  we  have  been  considering  will  show  that,  in 
the  evolution  of  such  a  system,  there  will  be  a  tendency 
for  the  planets  to  revolve  in  the  same  direction. 

It  is  easy  to  see  how,  in  the  contraction  of  the 
original  nebula,  there  must  have  been  a  strong  influence 
to  check  and  efface  any  movements  antagonistic  to 
the  general  direction  of  the  rotation  of  the  nebula. 
If  particles  revolve  in  a  direction  opposite  to  the  current 
pursued  by  the  majority  of  particles,  there  would  be 
collisions  and  frictions,  and  these  collisions  and  frictions 
will,  of  course,  find  expression  in  the  production  of 
equivalent  quantities  of  heat.  That  heat  will,  in  due 
course,  be  radiated  away  at  the  expense  of  the  energy 
of  the  system,  and  consequently,  so  long  as  any  contrary 
movements  exist,  there  will  be  an  exceptional  loss  of 
energy  from  this  cause.  Thus  the  energy  would  in- 
cessantly tend  to  decline.  As  the  shrinking  of  the  body 
proceeded  while  the  moment  of  momentum  would  have 
to  be  sustained,  this  would  incessantly  tend  more  and 
more  to  require  from  all  the  particles  a  movement  in 
the  same  direction. 

The  second  concord  of  the  planetary  system,  which 
is  implied  in  the  fact  that  all  the  planets  go  round  in 
the  same  direction,  need  not  therefore  surprise  us.  It 
is  a  consequence,  an  inevitable  consequence,  of  the  evo- 
lution of  that  system  from  the  great  primaeval  nebula. 
We  have  seen  that  it  would  be  excessively  improbable 
that  even  nine  or  ten  planets  should  revolve  round 
the  sun  in  the  same  direction,  if  the  directions  of  their 
movements  had  been  merely  decided  by  chance.  We 


322  THE   EARTHS   BEGINNING. 

have  seen  that  the  movements  of  the  hosts  of  planets, 
which  actually  form  our  system,  would  be  inconceivable,, 
unless   there  were  some  reason  for  those  movements. 
The  chances  against  such  an  arrangement  having  arisen 
without  some  predisposing  cause  is  so  vast  that,  even 
if  the  chances  were  infinite,  the  case  would  be  hardly 
strengthened.      But    once   we  grant   that   the  system 
originated  from  the  contraction  of  the  primaeval  nebula, 
dynamics  offers  ready  aid,  and  the  difficulty  vanishes. 
Not  only  do  we  see  most  excellent  reasons  why  all  the 
planets    should    revolve   in    the    same    direction;    we 
are   also  provided  with  illustrations  of  similar  evolu- 
tions in  progress  in   other  parts  of  the  universe ;  we 
learn   that   the   evolving  nebula,  however  erratic  may 
have  been  its  primitive  motion,  whatever  cross   cur- 
rents  may  have   agitated  it  in  the  early  phases  of  a 
possibly  violent  origin,  will  ultimately  attain  a  rotation 
uniform  in  direction.     As  the  evolution  proceeds,  the 
various  parts  of  the  nebula  draw  together  to  form  the 
planets   of  the  future  system,  and   the  planets  retain 
the  movement  possessed  by  their  component  particles. 
Thus  we  see  that  the  nebular  theory  not  only  extricates 
us  from  the  difficulty  of  trying  to  explain  something 
which  seemed  almost  infinitely  improbable,  but  it  also 
shows  why  no  other  disposition  of  the   motions  than 
that  which  we  actually  find  could  be  expected.     The 
nebular  theory  explains  to  us  why  there  is  no  excep- 
tion   to    that    fundamental    law    in   the   solar   system 
which   declares   that   the  orbits   of   the  planets    shall 
all  be  followed  in  the  same  direction. 

This  wonderful  agreement  in  the  movements  of  the 
planets,  which  we  have  called  the  second  concord,  thus 
affords  us  striking  evidence  of  the  general  truth  of 


MORE   PROOF   TO   COME.  323 

the  nebular  theory.  But  there  is  yet  a  third  concord 
in  the  solar  system  which,  like  the  other  two,  lends 
wonderful  corroboration  to  the  sublime  doctrine  of 
Kant  and  Laplace.  This  we  shall  consider  in  the  next 
chapter. 


CHAPTER    XVI. 

THE     THIRD     CONCORD. 

Rotations  of  the  Planets  on  their  Axes.— The  Older  Planets— No 
Information  about  Uranus  or  Neptune  or  the  Asteroids— The  Speed 
of  Rotation  is  Arbitrary  so  far  as  Kepler's  Laws  are  concerned— The 
Third  Concord — A  Remarkable  Unanimity — Kant's  Argument — 
Illustration  of  the  Rotation  of  the  Moon  on  its  Axis — How  the 
Nebular  Theory  explains  the  Rotation — The  Moon's  Evolution — 
Special  Action  of  Tides — The  Evolution  of  the  other  Satellites — 
The  case  of  Mars — Jupiter  and  Saturn  as  Miniatures  of  the  Solar 
System — Uranus  and  Neptune  offer  Difficulties. 

WE  have  seen  in  the  last  chapter  how  the  rotation  of 
the  sun  beat  time,  as  it  were,  for  the  planets,  by  giving 
to  them  an  indication  of  the  direction  in  which  the 
revolutions  round  the  sun  should  be  performed,  and 
we  have  observed  with  what  marvellous  unanimity  the 
planets  follow  the  precept  thus  given.  We  have  now 
to  consider  yet  another  concord,  which  has  perhaps  not 
the  great  numerical  strength  of  that  last  considered, 
but  is,  nevertheless,  worthy  of  our  most  special  attention. 
The  earth  revolves  about  an  axis  which  is  not  very 
far  from  being  perpendicular  to  the  principal  plane  to 
which  the  movements  of  the  solar  system  are  related. 
From  a  dynamical  point  of  view  it  would,  of  course, 
have  been  equally  possible  for  the  earth  to  revolve 


THE    CASE    OF    VENUS.  325 

on  its  axis  in  the  same  direction  as  the  rotation  of 
the  sun,  or  in  the  opposite  direction.  There  is  nothing 
so  far  as  the  welfare  of  man  is  concerned  to  make 
one  direction  of  rotation  preferable  to  the  other,  but, 
.as  a  matter  of  fact,  the  earth  does  turn  round  in  the 
same  way  as  the  sun  turns. 

Jupiter  also  turns  on  its  axis,  and  Jupiter  again, 
like  the  earth,  might  turn  either  with  the  sun  or  it 
might  turn  in  the  opposite  direction.  Here,  again,  we 
find  a  unanimity  between  the  earth  and  Jupiter :  they 
both  turn  in  the  same  direction,  and  that  is  the 
direction  in  which  the  sun  rotates.  The  same  may  be 
said  of  Mars,  and  the  same  may  be  said  of  Saturn.  In 
the  case  of  the  planets  Mercury  and  Venus  we  cannot 
speak  with  equal  definiteness  on  the  subject  of  their 
rotations  about  their  axes.  The  circumstances  of  these 
planets  are  such  that  there  are  great  difficulties  attend- 
ing the  exact  telescopic  determination  of  their  periods 
of  rotation.  The  widest  variations  appear  in  the  periods 
which  have  been  assigned.  It  has,  for  instance,  been 
believed  that  Venus  rotates  in  a  period  not  greatly 
differing  from  the  period  of  twenty-four  hours  in  which 
our  earth  revolves.  But  it  has  been  lately  supposed 
that  the  period  of  Venus  is  very  much  longer,  and  is  in 
fact  no  less  than  seven  months,  which  is,  indeed,  that  of 
the  revolution  of  Venus  about  the  sun.  According  to  this 
view,  Venus  rotates  round  the  sun  in  a  period  equal  to 
its  revolution.  If  this  be  so,  then  Venus  constantly 
turns  the  same  face  to  the  sun,  and  the  movement  of 
the  planet  would  thus  resemble  the  movement  of  the 
moon  around  the  earth.  As  a  matter  of  observation, 
the  question  must  still  be  considered  unsettled,  though 
there  are  sound  dynamical  reasons  for  believing  that  the 
22 


326  THE    EARTH'S    BEGINNING. 

long  period  is  much  more  probable  than  the  short  one. 
We  do  not  now  enter  into  this  question,  or  into  the 
still  more  difficult  matter  of  the  rotation  of  Mercury ; 
it  suffices  to  say  that  whichever  period  be  adopted  for 
either  of  these  planets  is  really  not  material  to  our 
present  argument.  In  both  cases  it  has  never  been 
doubted  that  the  direction  of  the  rotation  of  the  planets 
is  the  same  as  the  direction  in  which  Jupiter  and  Mars 
and  the  earth  rotate,  these  being  also  the  same  as  the 
direction  of  the  solar  rotation. 

As  to  the  rotations  of  Uranus  and  Neptune  about 
their  respective  axes,  the  telescope  can  show  us  nothing. 
The  remoteness  of  both  these  planets  is  such  that  we 
are  unable  to  discern  objects  on  their  discs  with  the 
defmiteness  that  would  be  required  if  we  desired  to 
watch  their  rotations.  We  have  also  no  information  as 
to  the  rotation  of  the  several  asteroids.  No  one,  I  think, 
will  doubt  that  each  of  these  small  planets,  equally 
with  the  large  planets,  does  rotate  about  its  axis ;  but 
it  is  impossible  for  us  to  say  so  from  actual  knowledge. 

But  undoubtedly  the  five  old  planets,  Mercury, 
Venus,  Mars,  Jupiter,  and  Saturn,  as  well  as  the  earth, 
all  rotate  in  the  same  direction  as  the  sun.  Each  planet 
might  rotate  twice  as  fast,  or  half  as  fast,  as  it  does  at 
present.  They  might  all  rotate  in  the  opposite  direc- 
tion from  that  in  which  they  do  now,  or  some  of 
them  might  go  in  one  direction,  and  some  in  the 
other,  with  every  variety  in  their  diurnal  periods,  while 
the  primary  condition  of  Kepler's  Laws  would  have 
still  been  complied  with.  We  may  also  note  that  the 
direction  in  which  the  rotation  takes  place  seems  quite 
immaterial  so  far  as  the  welfare  of  the  inhabitants  on 
these  planets  is  concerned. 


THE    THIRD    CONCORD.  327 

The  fact  that  the  planets  and  the  sun  have  this  third 
concord  demands  special  attention.  The  chance  that 
the  earth  should  rotate  in  the  same  direction  as  the  sun 
is,  of  course,  expressed  by  one-half.  It  is  easy  to  show, 
that  out  of  sixty-four  possible  arrangements  of  the 
directions  of  rotation  of  the  five  planets  and  the  earth, 
there  would  be  only  one  in  which  all  the  movements 
coincided  with  the  direction  of  the  rotation  of  the  sun. 
If,  therefore,  it  had  been  by  chance  that  the  direction 
of  these  motions  was  determined,  then  Nature  would 
have  taken  a  course  of  which  the  probability  was  only 
one  sixty-fourth.  No  doubt  this  figure  is  by  no  means 
so  large  as  those  which  expressed  the  probabilities  of 
the  other  planetary  concords ;  it  is,  however,  quite 
sufficient  to  convince  us  that  the  direction  of  the 
rotation  of  the  planets  on  their  axes  has  not  been 
determined  merely  by  the  operation  of  chance. 

We  are  to  see  if  there  is  any  physical  agent  by 
which  the  planets  have  been  forced  to  turn  round  in 
the  same  direction.  And  here  comes  in  one  of  those 
subtle  points  which  the  metaphysical  genius  of  Kant 
suggested.  Let  us  take  any  two  planets — say,  for 
instance,  the  earth  and  Jupiter — and  let  us  endeavour 
to  see  what  the  nature  of  the  agent  must  have  been 
which  has  operated  on  these  planets  so  as  to  make 
them  both  rotate  in  the  same  direction.  Kant  urged 
that  there  must  have  been  some  material  agent  working 
on  the  materials  in  Jupiter,  and  some  material  agent 
working  on  those  of  the  earth,  and  that  to  produce  the  like 
effect  in  each  planet  there  must  have  been  at  one  time 
a  material  connection  existing  between  that  body  which 
is  now  Jupiter  and  that  body  which  is  now  the  earth. 
In  like  manner  Kant  saw  this  material  connection 


328  THE    EARTH'S    BEGINNING. 

existing  between  the  other  planets  and  the  sun,  and 
thus  he  was  led  to  see  that  the  whole  material  of  our 
solar  system  must  once  have  formed  a  more  or  less 
continuous  object.  The  argument  is  a  delicate  one, 
but  it  seems  certainly  true  that  in  the  present  arrange- 
ment of  the  orbits  it  is  impossible  for  us  to  conceive 
how,  with  intervals  of  empty  space  between  the  tracks  of 
the  planets,  a  common  influence  can  have  been  exerted 
so  as  to  give  them  all  rotations  in  the  same  direction. 

The  nebular  theory  at  once  supplies  the  explanation 
of  the  unanimity  in  the  rotation  of  the  planets,  just 
as  it  supplied  the  explanation  of  the  unanimity  in  the 
directions  of  their  revolutions.  To  explain  the  rotation 
of  a  planet  on  its  axis,  let  us  imagine  that  one  portion 
of  the  contracting  nebula  has  acquired  exceptional 
density.  In  virtue  of  its  superior  attraction  it  absorbs 
more  and  more  material  from  the  adjacent  parts  of 
the  nebula,  and  this  will  ultimately  be  consolidated 
into  the  planet,  though  in  its  initial  stages  this  con- 
tracting matter  will  remain  part  of  the  nebula.  We 
have  shown  that  the  law  which  decrees  that  the 
moment  of  momentum  must  remain  constant  will 
require  that,  after  a  certain  advance  in  the  contrac- 
tion, all  the  parts  of  the  nebula  shall  rotate  in  the 
same  direction.  Thus  we  find  that  the  sun,  or  rather 
the  parts  of  the  nebula  that  are  to  form  the  sun, 
and  the  parts  that  are  to  form  the  planets  are  all 
turning  round  together. 

At  this  point  we  may  consider  a  geometrical 
principle  which,  though  really  quite  simple,  is  not 
always  easily  understood.  It  has  indeed  presented 
considerable  difficulty  to  many  people.  Suppose  that 
an  ordinary  card  is  laid  on  a  flat  board,  and  that,  with 


Fig.  51.— AN  ELONGATED  IRREGULAR  NEBULA  fn.g.c.  6992;  in  Cygnus). 

(Dr.  W.  E.  Wilson,  F.R.S  > 
(From  the  Astronomical  and  Physical  Researches  at  Darainona  Observatory.) 


330  THE   EARTH'S    BEGINNING. 

a  bradawl,  a  hole  is  made  through  the  card  into  the 
board.  The  hole  may  be  at  the  centre,  or  at  one  of 
the  corners,  or  a  little  way  in  from  one  of  the  edges, 
or  in  any  other  position  whatever  on  the  card.  Now 
suppose  that  a  postage  stamp  is  stuck  upon  the  card 
anywhere,  and  that  the  card  is  then  moved  around 
the  bradawl.  How  are  we  to  describe  the  motion  of 
that  postage  stamp  ?  It  would  certainly  be  revolving 
around  the  bradawl ;  but  this  motion  we  may  con- 
sider as  composed  of  two  others.  At  any  instant  we 
may  accurately  represent  the  movement  of  the  postage 
stamp  by  considering  that  its  centre  was  moving  in 
a  direction  perpendicular  to  the  line  joining  that  centre 
to  the  hole  made  by  the  bradawl,  and  that  it  also  had 
a  rotation  around  its  centre,  the  period  of  that  rotation 
being  just  the  same  as  the  time  the  card  would  take 
to  go  round  the  bradawl.  Thus  we  see  that  the  move- 
ment of  the  postage  stamp  contains  at  any  moment 
a  movement  of  translation  and  a  movement  of  rota- 
tion. 

We  may  illustrate  the  case  we  have  supposed  by 
the  movement  of  the  moon  around  the  earth.  If  the 
centre  of  the  earth  be  considered  to  be  at  the  centre 
of  rotation  the  moon  may  be  considered  to  be  in  the 
position  of  the  postage  stamp.  As  our  satellite  re- 
volves, the  same  side  of  the  moon  is  continually  turned 
towards  the  earth,  but  this  is  due  to  the  fact  that  the 
moon,  at  each  moment,  really  possesses  two  move- 
ments, namely,  a  movement  of  translation  of  its  centre, 
in  a  direction  perpendicular  to  the  line  from  the 
moon's  centre  to  the  earth's  centre,  coupled  with  a 
slow  rotation  of  the  moon  round  its  axis. 

The  contracting  nebula  we  may  liken  to  our  piece 


HOW  A   PLANET   GREW.  331 

of  cardboard,  the  stamp  will  represent  the  spot  in  which 
the  nebulous  material  has  contracted  to  form  the  planet, 
and  the  position  of  the  bradawl  is  the  centre  of  the  sun. 
As  we  have  seen  by  our  illustration,  the  nebulous 
planet  is  endowed  with  a  certain  movement  of  rotation, 
the  period  of  its  rotation  on  its  axis  being  equal  to 
that  of  its  revolution  around  the  centre ;  and  it  is 
important  also  to  notice  that  both  these  movements 
take  place  in  the  same  direction. 

Thus  we  see  from  the  nebular  theory  how  the 
primaeval  nebula,  in  the  course  of  its  contraction, 
originated  a  planet,  and  how  that  planet  was  also 
endowed  with  a  movement  of  rotation;  its  period  oi 
rotation  being  originally  equal  to  the  period  of  rota- 
tion of  the  whole  nebula.  This  explains  how  the  planet, 
or  rather  the  materials  which  are  to  form  the  future 
planet,  derived  from  the  nebula  their  movement  ol 
rotation,  which  must  have  been  extremely  slow  in  the 
beginning.  As  the  contraction  continued,  the  materials 
of  the  gradually  growing  globe  drew  themselves  to- 
gether, and  tended  to  become  separate  from  the  sur- 
rounding nebula.  At  length  the  time  arrived  when  the 
planet  became  sufficiently  isolated  from  the  rest  of 
the  nebula  to  permit  the  conservation  of  moment  of 
momentum  to  be  applied  to  it  individually.  Thus, 
though  the  rotation  was  at  first  excessively  slow,  yet, 
as  the  contraction  proceeded,  and  as  the  parts  of  the 
forming  planet  drew  themselves  closer  together,  in 
consequence  of  their  mutual  attractions,  it  became 
necessary  that  the  speed  with  which  these  parts  accom- 
plished their  revolutions  should  be  accelerated.  Thus, 
at  last,  when  the  planet  had  become  consolidated,  and 
when  consequently  the  mutual  distances  of  the  several 


332  THE    EARTH'S    BEGINNING. 

particles  constituting  the  planet  had  been  reduced  to 
but  a  fraction  of  what  those  distances  were  originally 
the  speed  of  the  planet's  rotation  had  become  enor- 
mously increased.  In  this  manner  we  learn  how,  from 
the  very  slow  rotation  which  the  nebulous  material 
had  at  first,  a  solid  planet  may  be  made  to  rotate  on 
its  axis  as  rapidly  as  the  planets  in  the  solar  system 
do  to-day. 

We  thus  find  that  the  third  concord,  namely,  the 
agreement  in  the  directions  of  the  planets'  rotations, 
is  a  further  strong  corroboration  of  the  nebular  theory. 
The  unanimity  of  all  these  various  movements  is  the 
dominant  characteristic  of  the  solar  system. 

But  this  third  concord,  derived  from  the  rotation 
of  the  planets,  may  be  yet  further  strengthened.  The 
movements  of  the  satellites,  which  accompany  so  many 
of  the  planets,  must  also  find  their  explanation  from 
the  primaeval  nebula.  The  circumstances  of  the 
satellites  are,  however,  different  in  the  different  cases. 

As  regards  the  moon,  the  theory  of  its  evolution 
is  now  well  known,  mainly  by  the  researches  of  Professor 
George  Darwin.  In  the  moon  there  appear  to  have 
been  causes  at  work  of  a  somewhat  special  kind.  We 
must  just  refer  to  what  is  well  known  with  regard  to 
the  history  of  the  moon.  Here,  again,  we  observe  the 
importance  of  the  principles  of  the  conservation  of 
moment  of  momentum.  As  the  moon  raises  tides  on 
the  ocean  surrounding  the  earth,  and  as  those  tides 
flow  around  the  globe,  they  cause  friction,  and  that 
friction  involves,  as  we  have  so  often  pointed  out, 
the  loss  of  energy  to  the  system.  Thus,  the  energy 
of  the  earth-moon  system  must  be  declining,  while  the 
moment  of  momentum  remains  constant.  Now  there 


THE    EARTH  AND    THE    MOON.  33$ 

are  only  two  sources  from  which  the  energy  can  be 
derived.  One  of  those  sources  is  that  due  to  the 
rotation  of  the  earth  on  its  axis.  The  other  is  due  to 
the  moon,  and  consists  of  two  parts,  namely,  the  energy 
arising  from  the  velocity  of  the  moon  in  its  orbit,  and 
the  energy  due  to  the  distance  by  which  the  earth  is 
separated  from  the  moon.  As  the  moon's  velocity 
depends  upon  its  distance,  we  cannot  view  these  two 
portions  as  independent.  They  are  connected  together, 
and  we  associate  them  into  one.  So  that  we  say  the 
total  energy  of  the  earth-moon  system  consists  partly 
of  that  due  to  the  rotation  of  the  earth  on  its  axis, 
and  partly  of  that  due  to  the  revolution  of  the  moon 
around  the  earth.  It  might  also  seem  that  we  ought 
to  add  to  this  the  energy  due  to  the  rotation  of  the  moon 
around  its  own  axis  ;  but  this  is  too  inconsiderable  to 
need  attention.  In  the  first  place,  the  moon  is  so  small 
that  even  if  it  rotated  as  rapidly  as  the  earth  the  energy 
due  to  the  rotation  would  not  be  important.  Seeing, 
however,  that  the  moon  has  for  the  rotation  on  its  axis 
a  period  of  between  twenty-seven  and  twenty-eight  days,, 
its  velocity  of  rotation  is  so  small  that,  for  this  reason 
also,  the  energy  of  rotation  would  be  inconsiderable. 
We  are,  therefore,  amply  justified  in  omitting  from  our 
present  consideration  the  energy  due  to  the  rotation 
of  the  moon  on  its  axis. 

The  energy  of  the  earth-moon  system  is  on  the  de- 
cline :  the  lost  energy  might  conceivably  be  drawn  from 
the  rotation  of  the  earth,  or  it  might  be  drawn  from  the 
revolution  of  the  moon,  or  it  might  be  drawn  from  both. 
If  it  were  drawn  from  the  revolution  of  the  moon,  that 
would  imply  that  the  moon  would  lose  some  of  its 
speed  or  some  of  its  distance,  or  in  any  case  that  the 


334  THE   EARTH'S    BEGINNING, 

moon  would  get  nearer  to  the  earth  and  revolve  more 
slowly,  the  speed  of  the  earth  being  on  this  supposition 
unaltered.  In  this  case,  the  moment  of  momentum  of 
the  earth  would  remain  the  same  as  before,  while  the 
moment  of  momentum  of  the  moon  would  be  lessened ; 
the  total  moment  of  momentum  would  therefore  have 
decreased,  but  this  we  have  seen  to  be  impossible.  It 
therefore  follows  that  the  energy  withdrawn  from  the 
earth-moon  system  is  not  to  be  obtained  at  the  expense 
of  the  revolution  of  the  moon. 

The  energy  must  therefore  be  obtained  at  the  ex- 
pense of  the  rotation  of  the  earth  on  its  axis.  But  if 
this  be  the  case,  the  speed  with  which  the  earth  rotates 
must  be  diminished ;  that  is  to  say,  the  length  of  the  day 
must  be  increased.  And  if  the  speed  of  the  earth's 
rotation  be  reduced,  that  means  that  the  amount  of 
moment  of  momentum  contributed  by  the  earth  is 
lessened.  But  the  total  quantity  of  moment  of  momen- 
tum must  be  sustained,  and  this  can  only  be  done  by 
making  the  moon  go  further  away  and  describe  a  larger 
orbit.  We  thus  see  that  in  consequence  of  the  tides  the 
length  of  the  day  must  be  increasing,  and  the  moon 
must  be  gradually  retreating.  Thus  we  find  that  at 
earlier  periods  the  moon's  distance  from  the  earth  must 
have  been  less  than  it  is  at  present,  and  the  further  we 
look  back  through  remote  periods  the  less  do  we  find 
the  distance  between  the  earth  and  the  moon.  Thus  we 
see  that  there  was  a  time  apparently,  when  the  materials 
of  the  moon  must  have  been  in  actual  contact  with 
the  materials  of  the  earth.  In  fact,  it  seems  quite 
possible  that  the  moon  may  have  been  a  portion  of 
the  earth,  broken  off  at  some  very  early  period,  while 
the  earth  was  still  in  a  liquid  state,  if  indeed  it  had 


THE    SATELLITES    OF    MARS.  335 

condensed  to  even  that  extent.  Thus  the  revolution  of 
the  moon  round  the  earth  is  hardly  to  be  used  as  an 
argument  in  favour  of  the  nebular  hypothesis.  The 
moon  is  indeed  a  consequence  of  the  earth's  rotation. 

The  satellites  of  Mars  offer  conditions  of  a  very 
different  kind,  though  here,  again,  tidal  influences  have 
been  so  important,  that  it  is  perhaps  questions  relating 
to  tides  that  are  illustrated  by  these  satellites  rather 
than  the  nebular  theory. 

A  remarkable  circumstance  may  be  noted  with 
regard  to  the  movements  of  the  satellites  of  Mars.  The 
inner  satellite  has  a  period  of  about  seven  and  a  half 
hours,  which  is  not  a  third  of  the  period  that  the  planet 
itself  takes  to  go  round  on  its  axis.  This  leads  to  a 
somewhat  curious  consequence.  The  tides  raised  on 
Mars  by  this  inner  satellite  would  certainly  tend  rather 
to  accelerate  the  rotation  of  the  planet  than  to  retard 
it ;  for  these  tides  must  course  round  the  planet  in 
the  direction  of  its  rotation,  but  with  a  speed  in 
excess  of  that  rotation.  Any  tidal  friction,  so  far  as 
this  satellite  is  concerned,  will  tend  to  augment  the 
velocity  of  the  planet's  rotation,  just  as  in  the  opposite 
case,  where  the  moon  raises  tides  on  the  earth,  it  is  the 
lagging  of  the  tides  behind  the  movement  due  to  the 
rotation  that  acts  as  a  brake,  and  tends  to  check  that 
speed.  If,  therefore,  Mars  is  accelerated  by  this  satellite, 
it  will  do  more  than  its  original  share  of  the  moment 
of  momentum  of  the  Martian  system;  it  is  therefore 
imperative  that  the  satellite  shall  do  less.  Accordingly, 
we  find  that  this  satellite  must  go  in  towards  the  planet. 
No  doubt  this  effect  is  much  complicated  by  the  influ- 
ence of  the  other  satellite  of  the  same  planet,  but  the 
illustration  may  suffice  to  show  that  if  the  satellites 


336  THE    EARTH'S    BEGINNING. 

of  the  earth  and  Mars  do  not  convey  to  us  much 
direct  evidence  with  regard  to  the  nebular  theory,  this  is 
largely  because  the  effect  of  the  tides  has  been  a  prepon- 
derating influence.  The  Martian  system  as  we  now  see 
it  has  acquired  its  characteristic  features  by  tidal 
influence,  so  that  the  more  simple  influences  which 
would  immediately  illustrate  the  nebular  theory  have 
become  hidden. 

As  to  the  satellites  of  Jupiter  and  Saturn,  the  circum- 
stances are  again  quite  different  from  those  that  we  find 
in  the  earth  and  in  Mars.  There  is  little  more  to  be 
said  with  regard  to  them  than  that  everything  that  they 
present  to  us  is  consistent  with  the  indications  of  the 
nebular  theory.  The  evolution  in  each  case  has  been 
a  reproduction  in  miniature  of  the  evolution  of  the  solar 
system. 

But  the  satellites  of  Uranus  and  Neptune  present,  it 
must  be  admitted,  the  greatest  stumbling  block  to  the 
acceptance  of  the  nebular  theory.  Both  as  to  the 
directions  in  which  they  move  and  as  to  the  planes  in 
which  their  orbits  lie,  it  must  be  admitted  that  the 
satellites  of  Uranus  are  distinctly  at  variance  with  what 
the  nebular  theory  would  suggest.  The  consideration  of 
this  subject  will  be  found  in  the  next  chapter. 


CHAPTER    XVII. 

OBJECTIONS  TO   THE   NEBULAR  THEORY. 

There  are  Difficulties  in  the  Nebular  Theory — The  General  Conformity 
of  the  Movements — Details  of  the  Uranian  Movements — The 
Anomaly  in  the  Satellite  of  Neptune — Where  the  Difficulty  Lies— 
The  Fundamental  Principle  which  Dynamics  Offers  for  our  Guidance 
— The  Immense  Contrast  between  the  Nebula  in  its  Original  Form  and 
its  Final  Form — Energy  that  could  be  Obtained  by  a  Re-arrangement 
of  our  System — Probable  Nature  of  the  Present  Change  in  the  Plane 
of  the  Orbits  of  the  Satellites  of  Uranus — The  Similar  Explanation 
in  the  Case  of  Neptune. 

No  one  will  deny  that  there  are  many  points  in  con- 
nection with  the  nebular  theory  which  still  offer  great 
difficulties.  We  shall  endeavour  to  consider  the  most 
formidable  of  these  in  this  chapter.  They  are  certain 
anomalous  phenomena  presented  by  the  planets  Uranus 
and  Neptune. 

The  satellites  which  attend  upon  the  planets  exhibit 
a  general  conformity  with  those  movements  of  the 
planets  themselves  on  which  we  have  dwelt  in  Chapters 
XIV.,  XV.,  XVI.  The  planes  in  which  the  orbits  of  the 
satellites  are  contained  are  usually  not  much  inclined  to 
the  plane  of  the  ecliptic,  and  the  directions  in  which  the 
satellites  revolve  also  agree  with  the  general  direction  of 
the  planetary  movement.  We  find  these  conditions  in 


338  THE   EARTH'S    BEGINNING. 

the  one  satellite  of  the  earth,  in  the  two  satellites  of 
Mars,  in  the  five  satellites  of  Jupiter,  in  the  eight  or 
nine  satellites  of  Saturn ;  but,  when  we  come  to  Uranus 
and  Neptune,  the  two  outermost  planets,  we  observe 
a  striking  but  most  instructive  violation  of  the  laws 
which  we  have  found  so  consistently  prevailing  in  the 
other  parts  of  the  solar  system. 

Let  me  first  mention  the  special  circumstances  of 
Uranus.  It  is  now  known  that  this  planet  has  four 
satellites.  Of  these,  Titania  and  Oberon  were  both  dis- 
covered by  Sir  William  Herschel  on  January  llth, 
1787.  The  two  remaining  satellites,  named  Ariel  and 
Umbriel,  were  not  discovered  for  more  than  half  a 
century  later  by  Mr.  Lassell,  on  October  24th,  1851. 
It  is,  however,  just  possible  that  they  were  previously 
seen  by  Sir  William  Herschel. 

The  innermost  of  the  four  satellites,  Ariel,  accom- 
plishes a  revolution  in  a  day  and  a  half.  Umbriel  goes 
round  in  four  days  and  three  hours,  Titania  in  eight  days 
and  seventeen  hours,  and  Oberon  in  thirteen  days  and 
eleven  hours.  We  have  already  mentioned  how  the 
investigations  of  Newcomb  show  that  these  four  satel- 
lites of  Uranus  revolve  in  the  same  direction  and  in 
the  same  plane;  but  this  plane,  instead  of  lying  in  or 
near  the  ecliptic,  is  very  nearly  perpendicular  thereto, 
the  actual  angle  being  eighty- three  degrees.  This  is 
one  of  the  features  in  which  the  satellites  of  Uranus 
are  in  startling  disobedience  to  the  laws  which  have 
been  so  rigidly  observed  in  most  other  parts  of  the 
system.  But  there  is  also  a  second  anomaly.  The 
direction  in  which  the  satellites  move,  when  projected 
on  the  plane  of  the  ecliptic,  is  found  to  be  opposite 
to  the  universal  direction  in  which  all  the  other 


THE    SATELLITES    OF    URANUS.  339 

movements  in  the  solar  system  are  performed.  Of 
course  the  fact  that  the  plane  of  the  orbits  of  the  satel- 
lites lies  so  nearly  at  right  angles  to  the  plane  of  the 
ecliptic  detracts  somewhat  from  the  significance  of  this 
circumstance.  If  the  two  planes  were  absolutely  at 
right  angles,  there  would  be,  of  course,  no  projection  at 
all,  and,  in  the  actual  circumstances,  the  moment  of 
momentum,  when  projected,  loses  nine  teen-twentieths  of 
its  amount.  It  follows  that  in  the  actual  position  of 
the  plane  the  abnormal  direction  in  which  the  satellites 
are  moving  is  not  very  material. 

It  must  be  admitted  that,  in  respect  both  of  the 
position  of  the  plane  of  their  orbits  and  the  direction 
of  their  movements,  the  satellites  of  Uranus  are  in 
marked  contrast  to  what  the  nebular  theory  might 
have  led  us  to  expect.  If  the  orbits  of  those  satellites 
had  all  lain  close  to  the  plane  of  the  ecliptic,  and  if 
the  direction  in  which  the  satellites  revolved  had  also 
conspired  with  that  of  the  revolution  of  Uranus  round 
the  sun,  and  with  all  the  other  hundreds  of  movements 
which  are  in  the  same  direction,  there  can  be  no  doubt 
that  we  should  in  this  place  have  been  appealing  to 
the  satellites  of  Uranus  as  confirmatory  evidence  of  the 
truth  of  the  nebular  theory.  The  fact  that  they  move 
in  a  manner  so  totally  at  variance  with  what  might  have 
been  expected  cannot  therefore  be  overlooked. 

Neptune,  the  outermost  planet  of  our  system,  pre- 
sents us  also  with  difficulties  of  an  analogous  character. 
So  far  as  the  orbit  of  Neptune  itself  is  concerned,  it 
agrees  entirely  with  the  general  planetary  convention ; 
the  inclination  of  that  orbit  to  the  plane  of  the  ec- 
liptic is  no  more  than  six  degrees,  and  the  direction 
in  which  the  outermost  planet  revolves  round  the 


340  THE   EARTH'S    BEGINNING. 

frontier  of  our  system  is  not  different  from  the  direc- 
tions in  which  all  the  other  planets  revolve.  We  know 
nothing  about  the  axis  of  rotation  of  Neptune  except 
that  it  may  be  reasonably  presumed  to  be  in  the  same 
plane  as  the  movement  of  its  satellite.  On  October 
10th,  1846,  Lassell,  with  the  help  of  his  great  tele- 
scope, suspected  the  existence  of  a  satellite  to  Neptune, 
and  he  announced  it  definitely  on  July  7th,  1847. 
We  are  indebted  to  Newcomb  for  a  careful  investiga- 
tion of  the  orbit  of  this  satellite.  It  moves  in  a  track 
which  is  practically  circular,  and  it  requires  about  five 
days  and  twenty-one  hours  to  accomplish  each  revolu- 
tion. Its  inclination  to  the  ecliptic  is  not  so  anomalous 
as  in  the  case  of  Uranus,  the  inclination  being  in  this 
case  not  more  than  thirty-five  degrees.  This  is  not  much 
greater  than  the  inclinations  of  the  orbits  of  some  of 
the  asteroids,  and  it  might  have  passed  without  much 
comment  had  it  not  been  for  the  circumstance  that 
the  direction  of  motion  of  the  satellite  in  this  track 
is  antagonistic  to  all  the  other  movements  in  the  solar 
system.  This  is  indeed  a  more  startling  fact  in  some 
respects  than  the  movements  of  the  satellites  of  Uranus, 
for,  as  we  pointed  out,  the  plane  of  the  orbits  of  the 
satellites  of  Uranus  is  so  nearly  perpendicular  to  the 
plane  of  the  ecliptic  that '  the  direction  of  the  movement 
could  not  be  held  to  be  of  much  significance.  The 
satellite  of  Neptune,  having  an  orbital  inclination  barely 
more  than  a  third  of  a  right  angle,  exhibits  a  retrograde 
movement  which  is  in  some  respects  the  most  anoma- 
lous feature  in  the  solar  system. 

These  circumstances  connected  with  the  satellites 
of  Uranus  and  Neptune  have  been  sometimes  brought 
forward  as  arguments  against  the  nebular  theory. 


ATTACKING    DIFFICULTIES.  341 

What  Laplace  would  have  said  to  them  we  can  only 
conjecture,  for,  at  the  time  he  brought  out  his  theory, 
Neptune  was  entirely  unknown,  and  none  of  the 
satellites  of  Uranus  had  been  observed.  But  it  has 
sometimes  been  urged  that  the  movements  of  these 
two  systems  are  inconsistent  with  the  principles  of  the 
fiebular  theory,  and  that,  therefore,  the  nebular  theory 
must  be  abandoned.  I  have  no  desire  to  minimise 
the  difficulties,  but  I  think  that  the  considerations  to 
which  I  now  invite  attention  may  help  to  lessen  them, 
even  if  they  do  not  altogether  remove  them.  I  trust, 
at  least,  we  may  be  able  to  show  that  even  these 
anomalous  movements  are  not  incompatible  with  the 
acceptance  of  the  account  of  the  origin  of  our  solar 
system  given  by  the  nebular  theory. 

The  primaeval  nebula  may  be  regarded  as  chaotic  in 
its  earliest  stages ;  perhaps  it  was  like  the  nebulous 
wisps  in  Fig.  51.  It  was  chaotic  in  the  arrangement 
of  the  material  of  which  it  is  formed,  and  in  the 
movements  of  that  material.  Before  a  disorganised 
nebula  can  become  evolved  into  a  nebula  with  any 
definite  form  like  that  in  Fig.  52,  or  into  anything 
resembling  a  solar  system,  an  immense  period  of  time 
must  elapse,  and  during  that  time  the  operation  of  the 
laws  of  dynamics  gradually  impresses  certain  well-marked 
features  on  the  nebula,  and  disposes  it  to  assume  an 
orderly  form.  We  have  explained  that  no  matter  how 
the  nebula  originated,  or  no  matter  what  may  have  been 
the  irregularities  in  its  extent  or  distribution,  and  no 
matter  how  diverse  may  have  been  the  agitations  of 
its  various  parts,  the  principles  of  dynamics  assure  us 
that  each  such  nebula  must,  for  all  time,  stand  in 
some  special  relation  to  a  certain  particular  plane.  The 

23 


342  THE    EARTH'S    BEGINNING. 

moment  of  momentum  which  the  nebula  has  with 
respect  to  this  plane,  exceeds  the  moment  of  momentum 
that  it  has  with  respect  to  any  other  plane.  We  have 
pointed  out  how,  notwithstanding  the  vicissitudes  and 
transformations  to  which,  in  the  course  of  illimitable 
ages,  the  nebula  must  submit,  its  moment  of  momentum 
relatively  to  this  plane  will  remain  absolutely  unaltered. 
We  have  shown  how  the  energy  of  the  nebula  becomes 
gradually  exhausted.  The  collisions  between  various 
particles,  the  frictions  that  will  necessarily  arise,  and  the 
actions  which  we  may  sufficiently  describe  by  saying 
that  they  are  of  a  tidal  character,  will  all  result  in 
the  transformation  of  energy  into  heat.  This  heat  is 
radiated  away  and  lost,  and  there  is  a  corresponding 
decline  in  the  energy  of  the  system.  To  preserve  its 
moment  of  momentum  unaltered  in  the  course  of  ages, 
notwithstanding  the  continuous  reduction  of  energy, 
the  materials  of  the  nebula  will  ever  find  themselves 
more  and  more  approximating  to  the  plane,  and  will 
ever  find  themselves  more  and  more  compelled  to  re- 
volve in  the  same  direction.  If  the  original  size  of  the 
nebula  be  compared  with  the  area  of  the  Atlantic  Ocean, 
the  condensed  form  which  the  nebula  may  ultimately 
assume  may  be  no  larger  than  a  coral  island.  If  the 
nett  moment  of  momentum,  diffused  over  the  space  as 
large  as  the  ocean,  has  still  to  be  preserved  in  the  space 
as  large  as  the  island,  we  need  not  be  surprised  that 
the  spin  of  the  system  in  its  condensed  form  is  its 
dominating  characteristic. 

In  the  evolution  of  our  solar  system  from  the 
primaeval  nebula,  this  operation  of  reducing  the  move- 
ments to  the  same  plane  and  of  requiring  that  all  the 
movements  shall  take  place  in  the  same  direction, 


THE    SYSTEM   NOT  PERFECT.  343 

having  had  play  for  unmeasured  ages,  has  in  the  main 
accomplished  its  end.  All  the  important  bodies  of  the 
system  do  go  round  in  the  same  direction ;  that  much, 
at  least,  has  been  attained.  All  of  them  also  go  round 
in  planes  which  are  nearly  coincident,  but,  as  we  have 
already  noted,  they  are  not  yet  absolutely  coincident. 
The  greatest  planets  have,  however,  very  nearly  become 
reconciled,  so  far  as  the  planes  of  their  orbits  are 
concerned,  to  the  condition  which  dynamics  imposes. 
The  same  is  true  of  the  rotation  of  the  sun  on  its  axis. 
That  axis  is  inclined  at  an  angle  of  eighty-three  degrees 
to  the  plane  of  the  ecliptic,  so  that  the  sun's  equator 
would  have  to  be  shifted  only  through  an  angle  no 
greater  than  seven  degrees,  if  it  were  to  be  placed  in  the 
plane  in  which  it  should  be  situated,  if  the  condition  of 
the  smallest  quantity  of  energy  for  a  given  amount  of 
moment  of  momentum  was  to  be  realised.  We  find  a 
greater  discrepancy  in  the  plane  of  the  earth's  equator. 
This  is  inclined  by  about  twenty-three  degrees  to  the 
plane  of  the  ecliptic.  Here  there  is  some  energy  which 
might  yet  be  expended  without  a  diminution  of  the 
amount  of  moment  of  momentum  in  the  system ;  for  if 
the  earth's  axis  were  to  be  made  perpendicular  to  the 
plane  of  the  ecliptic,  then  the  velocity  of  rotation  of  the 
earth  about  its  axis  might  undergo  a  corresponding 
abatement,  and  yet  keep  up  the  requisite  moment  of 
momentum.  We  thus  see  that  even  with  the  older 
planets  the  conditions  which  would  be  enforced,  if  the 
moment  of  momentum  was  to  be  sustained  with  the 
least  quantity  of  energy,  are  not  absolutely  complied 
with;  which  simply  means  that  there  has  not  yet 
been  time  enough  for  our  system  to  arrive  at  the 
perfect  state,  to  which  it  must  be  approximating. 


344  TEE   EARTH'S    BEGINNING. 

If  we  have  found  that  in  the  rotations  of  the  earth 
and  of  the  sun,  and  in  the  revolutions  of  the  planets 
round  the  sun,  the  conditions  ultimately  aimed  at 
have  not  yet  been  reached,  why  should  we  feel  surprised 
that  in  the  outer  planets  of  our  system,  Uranus  and 
Neptune,  the  conditions  which  evolution  tends  to  pro- 
duce have  not  yet  been  fully  attained?  That  the 
operation  of  the  conservation  of  moment  of  momentum 
is  in  progress  in  the  internal  economy  of  the  Uranian 
system,  we  have  already  had  occasion  to  explain  in 
Chapter  XL  The  fact  which  Newcomb  demonstrated, 
that  the  four  satellites  revolve  in  the  same  plane,  can 
only  be  accounted  for  by  the  supposition  that  in  that 
system  the  conservation  of  moment  of  momentum,  with 
declining  energy,  has  gradually  imposed  this  condition 
on  the  system  -belonging  to  Uranus.  With  reference  to 
the  position  of  the  plane  of  the  satellites,  in  the  case  of 
Uranus  and  Neptune,  we  would  say,  that  though  at 
present  their  arrangement  appears  anomalous,  it  will 
probably  not  always  remain  so.  The  fact  that  the 
satellites  of  Uranus  are  in  a  plane  nearly  perpendicular 
to  the  plane  of  the  ecliptic  really  implies  that  there 
is  a  certain  amount  of  energy  still  disposable  in  our 
system,  if  by  readjustment  of  the  plane  of  the  Uranian 
satellites  the  necessary  moment  of  momentum  in  the 
system  is  still  preserved. 

The  laws  of  dynamics  tell  us  that  the  orbits  of 
planets  must  be  gradually,  if  with  excessive  slowness, 
tending  still  further  to  the  same  plane.  In  this  process 
energy  can  be  expended  by  the  system,  while  the 
moment  of  momentum  is  unabated.  We  can  at  least 
suggest  what  seems  to  be  at  this  moment  in  progress  in 
the  system  belonging  to  Uranus.  It  will  readily  be 


CHANGES    GOING    ON. 


345 


Fig.  52. — TWO-BRANCHED  SPIRAL  (n.g.c.  7479;  in  Pegasus). 

(Lick  Observatory.) 

admitted  that  there  may  be  a  difficulty  in  seeing  how 
the  movement  of  a  planet,  which  is  going  in  the  wrong 
direction,  could  be  stopped  and  turned  into  the  right 
direction.  But  we  need  not  suppose  that  so  violent  a 
change  as  this  would  imply  is  to  be  expected  in  our 
system.  We  are  quite  accustomed  to  find  the  planes 
of  the  orbits  of  all  planets  in  gradual  movement.  The 
plane  containing  the  orbits  of  the  four  satellites  of 
Uranus  is  at  this  moment  probably  moving  gradually 
upwards.  It  will  in  due  course  become  actually  at  right 


346  THE    EARTH'S    BEGINNING. 

angles  to  the  ecliptic,  and  we  may  then  reasonably 
assume  that  it  will  advance  further  in  the  same  direc- 
tion. At  the  moment  the  right  angle  is  passed,  this  con- 
tinuous movement  will  have  the  effect  of  changing  the 
directions  of  the  satellites'  movement  from  retrograde 
to  direct.  The  present  anomaly  will  then  tend  to 
become  evanescent,  for,  as  the  exhaustion  of  the  energy 
continues,  the  planes  of  the  satellites  of  Uranus  will 
gradually  come  into  conformity  with  the  plane  of  the 
ecliptic. 

We  make  no  doubt  that  there  may  be  a  similar 
explanation  of  the  movements  of  the  satellite  of 
Neptune.  The  inclination  of  the  plane  of  the  orbit  of 
the  satellite  to  the  ecliptic  is  probably  now  increasing. 
It  will  ultimately  come  to  be  at  right  angles  thereto,  and 
then  the  next  advance  of  the  plane  will  convert,  by  a 
continuous  action,  the  retrograde  motion  of  the  satellite, 
at  present  so  disconcerting,  into  a  direct  motion.  The 
change  of  the  plane  will  still  continue  until  it,  too,  may 
ultimately  coalesce  with  the  ecliptic. 

The  fact  appears  to  be,  that  though  an  enormous 
quantity  of  energy  must  have  been  lost  by  radiation 
from  our  system  during  the  illimitable  ages  through 
which  the  evolution  has  been  running  its  course,  the 
disposable  energy  is  not  yet  quite  exhausted.  There  are 
certain  adjustments  in  our  system  which  may  still  be 
made  and  which  will  allow  of  yet  further  radiation  of 
energy,  while  still  preserving  sufficient  to  keep  up  the 
necessary  moment  .of  momentum.  It  seems  obvious 
that  the  system  is  tending  towards  a  condition  in  which 
the  planes  of  all  the  orbits  shall  be  coincident,  and  in 
which  all  the  directions  shall  be  absolutely  unanimous. 
If  we  were  at  once  to  alter  the  system  by  moving  all  the 


NO    DISPROOF    OF    THE    THEORY.  347 

orbits  into  the  plane  of  the  ecliptic,  but  making  no 
change  in  the  dimensions  of  those  orbits,  or  the 
velocities  concerned ;  if  we  were  also  to  adjust  the 
rotations  of  the  earth,  as  well  as  of  the  other  planets,  so 
that  all  the  axes  of  rotation  should  be  perpendicular  to 
the  plane  of  the  ecliptic  ;  if  we  were  to  turn  the  plane  of 
the  satellites  of  Uranus  through  that  angle  of  97°, 
which  would  suffice  at  the  same  time  to  bring  it  into 
coincidence  with  the  ecliptic,  and  lay  the  movements  of 
the  satellites  in  the  right  direction ;  if  we  were  also  to 
turn  the  orbit  of  the  satellite  of  Neptune  through  145°, 
thus  bringing  that  orbit  to  coincide  with  the  plane  oi 
the  ecliptic,  in  such  a  manner  that  the  direction  oi 
the  movement  of  the  satellite  of  Neptune  conspired  with 
all  the  other  movements  of  the  system,  then  this  re- 
arrangement of  the  system  would  increase  the  moment 
of  momentum,  while  the  quantity  of  energy  was  not 
altered.  But  this  is  the  same  thing  as  saying  that  some 
energy  yet  remains  to  be  disposed  of,  while  the  system 
still  preserves  the  requisite  moment  of  momentum. 

The  conclusion  we  come  to  may  be  thus  expressed: 
the  movements  of  the  satellites  of  Uranus  and  Neptune 
do  not  disprove  the  nebular  hypothesis.  They  rather 
illustrate  the  fact  that  the  great  evolution  which 
has  wrought  the  solar  system  into  form  has  not  yet 
finished  its  work;  it  is  still  in  progress.  The  work  is 
very  nearly  done,  and  when  that  work  shall  have  been 
completed,  the  satellites  of  Uranus  and  Neptune  will 
no  longer  be  dissociated  from  the  general  concord 


CHAPTER    XVIII. 

THE   BEGINNING   OF   THE   NEBULA. 

Nebula  not  of  Infinite  Duration— 8,300  Coal  Units  was  the  Total 
Energy  of  the  System — 460  Miles  a  Second — Solar  Nebula  from 
a  Collision — What  we  Know  as  to  the  Colliding  Bodies — Probability 
of  Celestial  Collisions — Multitudes  of  Dark  Objects — New  Star  in 
Perseus — Characteristics  of  New  Stars — Incandescent  Hydrogen — 
The  Ruby  in  the  Spectrum — Photographs  of  the  Spectrum — • 
Rarity  of  a  Collision  on  a  Scale  Adequate  to  a  Solar  System. 

WHATEVER  may  have  been  the  antiquity  of  the  actual 
elements  that  formed  the  primaeval  nebula  from  which 
the  solar  system  has  been  evolved,  the  nebula  itself 
has  certainly  not  been  of  infinite  duration.  The 
question  then  arises  as  to  what  has  been  the  origin 
of  the  nebula  as  such,  or  rather  by  what  agency  the 
material  from  which  the  nebula  was  formed  underwent 
so  radical  a  transformation  from  its  previous  condition 
as  to  be  changed  into  that  glowing  object  which  we 
have  considered  so  frequently  in  this  book  We  have  to 
explain  how,  by  the  operation  of  natural  causes,  a  dark 
body  can  be  transformed  into  a  glowing  nebula. 

Let  us  first  estimate  what  the  quantity  of  energy 
in  that  system  is.  The  sun  has  been  pouring  forth  heat 
tor  illimitable  ages,  and  will  doubtless  continue  to  pour 


THE    END    OF    THE    SUN.  349 

forth  heat  for  millions  of  years  to  come.  But  the 
destiny  which  awaits  the  sun,  though  it  may  be  pro- 
tracted, yet  cannot  be  averted.  The  sun  will  go  on 
pouring  forth  its  heat  and  gradually  shrinking.  The 
time  will  come  at  last  when  the  radius  of  the  sun  will 
have  appreciably  decreased,  and  when  once  it  has 
assumed  a  density  corresponding  to  a  solid  state  its 
history  as  a  radiant  globe  will  be  approaching  its  close. 
A  period  of  insignificant  extent,  a  century  or  less,  will 
then  suffice  for  that  solid  globe  to  cool  down  so  as  to 
be  no  longer  an  efficient  source  of  light  and  heat.  We 
shall  assume  that  when  the  sun  has  ultimately  become 
solid  and  cold,  and  when  it  is  no  longer  the  life  and 
light  of  our  system,  it  will  have  attained  a  mean  density 
of  21 '5,  which  we  have  chosen  because  that  is  the  density 
of  platinum,  the  heaviest  substance  known.  In  all 
probability  the  solar  density  will  never  become  so  great 
as  this,  but  to  include  the  most  extreme  case  in  our 
argument  1  am  making  the  assumption  in  the  form 
stated.  We  are  now  to  estimate  what  will  have  been 
the  total  energy  that  the  sun  has  radiated  from  the 
moment  when  as  an  indefinitely  great  nebula  it  first 
began  to  radiate  at  all,  down  to  that  moment  in  the 
future  when,  having  shrunk  to  the  density  of  platinum, 
and  having  parted  with  all  its  heat,  the  solar  radiation 
is  at  an  end. 

In  the  beginning  of  the  evolutionary  history  the 
sun  was  a  nebula,  which  we  have  supposed  to  extend 
in  every  direction  to  an  indefinitely  great  distance. 
The  system  has  resulted  from  the  contraction  of  that 
nebula,  and  the  energy  liberated  in  that  contraction 
has  supplied  the  sun's  radiation.  We  calculate  (see 
Appendix)  the  energy  that  would  be  given  out  in 


350  THE   EARTH'S    BEGINNING. 

the  contraction  of  a  nebula  whose  materials  were 
originally  at  infinity,  and  which  ultimately  coalesced 
to  form  a  cold,  solid  globe  of  the  density  of  platinum, 
and  as  heavy  as  the  sun.  There  is  no  object  in 
attempting  to  express  this  quantity  of  energy  in  foot- 
pounds— the  figures  would  convey  no  distinct  impres- 
sion— we  shall  employ  the  coal-unit  explained  in 
Chapter  VI.  We  imagine  a  globe  of  coal  the  weight  of 
the  sun ;  then,  if  that  globe  of  coal  were  adequately 
supplied  with  oxygen,  it  would,  on  combustion,  give 
out  a  certain  amount  of  heat,  which  is  a  convenient 
unit  for  our  measurements.  It  is  demonstrated  that 
the  quantity  of  energy  given  out  by  the  contraction 
of  the  nebula  from  infinity,  to  this  globe  of  the  density 
of  platinum,  would  be  about  equal  to  the  quantity  of 
energy  which  would  be  produced  by  the  combustion  of 
8,300  globes  of  coal  as  heavy  as  the  sun,  an  adequate 
contribution  of  oxygen  being  supposed  to  be  supplied. 
This  expresses  the  original  endowment  of  energy  in  the 
solar  system,  or  rather  a  major  limit  to  that  endow- 
ment ;  it  shows  that  the  solar  system  can  never  have 
developed  more  energy  by  contraction  than  that  which 
could  be  produced  by  the  combustion  of  8,300  globes 
of  coal  as  heavy  as  the  sun.  We  may  mention  that 
of  this  great  endowment  of  energy  an  amount  which 
is  rather  less  than  half  (3,400)  has  been  already  ex- 
pended, so  that  rather  more  than  half  of  the  sun's 
career  as  a  radiant  globe  may  yet  have  to  be  run. 

We  can  also  express  the  total  energy  of  the  solar 
system  in  a  different  manner.  AVe  shall  consider  what 
must  be  the  velocity  of  the  sun,  so  that  the  energy 
that  it  will  possess,  in  virtue  of  that  velocity,  shall  be 
equal  to  the  energy  which  could  be  produced  by  the 


HOW    THE    NEBULA    BEGAN.  351 

combustion  of  8,300  globes  of  coal  ot  the  same  weight. 
This  calculation  is  very  much  simplified  by  making  use 
of  a  principle  which  we  have  already  stated  and  applied 
in  Chapter  V.  We  have  shown  that  if  a  piece  of  coal 
be  animated  with  a  velocity  of  five  miles  a  second, 
the  energy  it  possesses  in  virtue  of  that  motion  is  equal 
to  the  energy  produced  by  the  coal  in  the  act  of  com- 
bustion. If  a  body  were  moving  at  the  rate  of,  let 
us  say,  100  miles  a  second — its  speed  being  then 
twenty  times  as  great  as  the  particular  speed  just 
mentioned — its  energy,  which  depends  on  the  square 
of  the  velocity,  would  be  400  times  as  much  as 
would  be  produced  by  the  burning  of  a  piece  of  coal 
equal  to  it  in  weight.  We  can  easily  calculate  that 
if  the  sun  were  moving  at  a  speed  of  460  miles 
a  second,  it  would  possess,  in  virtue  of  its  motion,  as 
much  energy  as  would  be  generated  by  the  contrac- 
tion of  the  primaeval  nebula  from  infinity  down  to  a 
globe  of  the  density  of  platinum. 

It  is  thus  easy  to  form  a  supposition  as  to  how 
the  nebula  constituting  our  solar  system  may  have 
come  into  being ;  most  probably  it  originated  in  this 
way.  Let  us  suppose  that  two  masses,  either  dark 
or  bright,  either  hot  or  of  the  temperature  of  space, 
or  the  temperature  of  frozen  air,  were  moving  with 
speeds  of  460  miles  a  second.  No  doubt  the  velocities 
we  are  here  postulating  are  very  high  velocities,  but 
they  are  not  unprecedentedly  high.  We  know  of  stars 
which  at  this  present  moment  move  quite  as  fast,  so 
that  there  is  nothing  unreasonable  in  our  supposition 
so  far  as  the  velocities  are  concerned.  Let  us  suppose 
that  each  of  these  bodies  had  a  mass  which  is  half  that 
of  our  present  solar  system.  If  these  two  bodies  dashed 


352  THE    EARTH'S   BEGINNING. 

into  collision,  when  moving  from  opposite  directions, 
the  effect  of  the  blow  would  be  to  transform  the  energy 
into  heat.  That  heat  would  be  so  great  that  it  would 
be  sufficient  not  alone  to  render  these  globes  red-hot 
and  white-hot,  but  even  to  fuse  them — nay,  further,  to 
drive  them  into  vapour,  even  to  a  vapour  which  might 
expand  to  an  enormously  great  distance.  In  other 
words,  it  is  quite  conceivable  that  a  collision  of  two 
such  masses  as  we  have  here  supposed  might  be 
adequate  to  the  formation  of  a  nebula  such  as  that 
one  which  in  the  lapse  of  indefinite  ages  has  shaped 
itself  into  the  solar  system. 

Before  the  collision,  which  resulted  in  the  formation 
of  the  nebula,  each  of  these  bodies,  or  rather  their 
centres  of  gravity,  would  be  moving  in  what  may  be 
regarded  for  the  moment  as  straight  lines,  and  a 
plane  through  those  two  straight  lines  will  be  a  plane 
which  for  ever  afterwards  will  stand  in  important 
relation  to  the  system.  It  will  be,  in  fact,  that 
principal  plane  of  which  we  have  so  often  spoken. 

As  those  two  bodies  met  they  would  possess  a 
certain  moment  of  momentum,  and  this  moment  of 
momentum  would  remain  for  ever  unaltered,  no  matter 
what  may  be  the  future  vicissitudes  of  the  system. 

For  the  sake  of  simplicity  in  describing  what  has 
occurred,  we  have  spoken  as  if  the  two  bodies  were 
of  equal  mass,  and,  moving  with  equal  velocities  from 
opposite  points  of  the  heavens,  dashed  into  collision. 
But  what  actually  happens  cannot  have  been  quite  so 
symmetrical.  There  is  one  feature  in  the  solar  system 
which  absolutely  proves  that  the  collision  cannot  have 
taken  place  precisely  in  the  way  we  have  laid  down. 
If  it  had  happened  that  two  equal  masses  had  been 


WHAT  IS   PROBABLE.  353 


Fig.  53.— CLUSTER  WITH  STARS  OF  17TH  MAGNITUDE  (n.g.c.  6705  ;  in 

Antinous). 
(Photographed  by  Dr.  Isaac  Roberts,  F.R.S.) 

hurled  into  collision  with  equal  velocities  from  pre- 
cisely opposite  directions,  then  there  could  have  been  no 
resultant  moment  of  momentum.  From  the  principle 
of  the  conservation  of  moment  of  momentum,  we  can 
see  that,  if  absent  in  the  beginning,  it  could  never 
originate  later.  As,  however,  we  have  a  large  moment 
of  momentum  in  the  movements  of  the  planets  and  the 
sun,  it  is  certain  that  the  collision  cannot  have  taken 
place  in  a  manner  quite  so  simple. 

The  probabilities  of  the  case  show  that  it  is  almost 
infinitely  unlikely  that  two  bodies  of  equal  dimensions, 
and  moving  with  equal  velocities  in  opposite  directions, 
should  come  squarely  into  collision.  It  would  be  much 
more  likely  that  the  bodies  should  be  not  of  the  same 
size,  not  moving  with  the  same  velocity,  and  should 
collide  partially  rather  than  squarely.  The  collision 
may  have  been,  in  fact,  little  more  than  a  graze.  The 


354  THE   EARTH'S    BEGINNING. 

probabilities  of  the  case  are  such  as  to  show  that  what 
actually  occurred  was  a  collision  between  two  unequal 
masses,  which  were  moving  in  directions  inclined  to 
each  other  and  with  different  velocities.  It  is  easy  to 
show  that,  granted  sufficiently  great  velocities,  an  im- 
pact which  fell  far  short  of  direct  collision  might  still 
produce  enough  heat  to  transform  the  whole  solar 
system  into  vapour. 

The  circumstances  which  would  naturally  accompany 
so  transcendent  an  incident  wTill  also  go  far  to  account 
for  a  difficulty  which  has  been  often  felt  with  regard 
to  the  evolution  of  the  system  from  a  nebula.  Were 
such  a  collision  to  take  place  we  should  certainly  not 
expect  that  the  resulting  nebulous  mass,  the  product 
of  a  shock  of  such  stupendous  violence,  would  be  a 
homogeneous  or  symmetrical  object.  Portions  of  the 
colliding  body  would  become  more  highly  heated  than 
others ;  portions  of  the  bodies  would  not  be  so  com- 
pletely transformed  into  vapour  as  would  other  parts. 
There  would  thus  be  differences  in  the  nebula  at  the 
different  parts  of  its  mass.  This  non-homogeneity 
would  be  connected  with  the  formation  and  growth  of 
planets  in  the  different  parts  of  the  nebula. 

There  is  another  circumstance  connected  with  the 
movement  of  the  sun  which  should  here  be  mentioned. 
It  is  well  known  that  the  sun  has  a  velocity  which 
carries  it  on  through  space  at  the  rate  of  half  a  million 
miles  a  day.  In  this  movement  the  whole  solar  system, 
of  course,  participates.  This  movement  of  translation 
of  our  system  must  also  be  a  result  of  the  movements 
of  the  two  original  colliding  masses.  These  two  masses 
imparted  to  the  system,  which  resulted  from  their 
union,  both  the  lineal  velocity  with  which  it  advances. 


COLLISIONS.  355 

through   space,  and  also  that  moment  of  momentum 
which  is  of  such  vast  importance  in  the  theory. 

A  consideration  of  the  probabilities  of  the  case  make 
it  quite  certain  that  the  celestial  bodies  we  see  are 
as  nothing  compared  with  the  dark  bodies  we  do  not 
see.  The  stars  we  see  are  moving,  and  the  natural 
assumption  is  that  the  dark  objects  with  which  the 
heavens  teem  are  also  in  motion.  We  shall,  under 
these  conditions,  not  feel  any  insuperable  difficulty  in 
the  supposition  that  collisions  between  different  bodies 
in  the  heavens  may  have  taken  place  from  time  to 
time.  We  remember  that  these  bodies  are  moving  in 
all  directions,  and  at  extremely  high  velocities.  We 
are  quite  willing  to  grant  the  excessive  improbability 
that  any  two  bodies  particularly  specified  should 
come  into  collision.  Within  view  of  our  telescopes  we 
have,  however,  a  hundred  millions  of  stars,  and  if  we 
multiply  that  figure  even  by  millions,  it  will  still,  we 
may  well  suppose,  not  be  too  large  to  express  the 
number  of  bodies  which,  though  contained  within  the 
region  of  space  ranged  over  by  our  telescopes,  are 
still  totally  invisible.  In  these  circumstances,  we  may 
admit  that  occasional  collisions  are  not  impossible- 
Please  note  the  strength  which  the  argument  derives 
from  the  enormous  increase  in  our  estimate  of  the 
number  of  bodies,  when  we  include  the  dark  objects 
as  well  as  the  stars.  If  we  were  asked  whether  it 
would  ever  be  possible  for  two  bright  stars  to  come 
into  collision,  we  might  well  hesitate  about  the  answer. 
We  know,  of  course,  that  the  stars  have  proper  motions ; 
we  know,  too,  that  the  stars,  in  this  respect  unlike  the 
planets,  have  no  definite  directions  of  movement  under 
the  control  of  a  supreme  co-ordinating  attraction.  Some 


356  THE   EARTH'S    BEGINNING. 

stars  move  to  the  right,  and  some  to  the  left,  some 
one  way  and  some  another;  but  even  still,  notwith- 
standing their  great  number,  the  extent  of  space  is 
such  that  the  stars  keep  widely  apart,  and  thus 
collisions  can  hardly  be  expected  to  take  place,  unless 
perhaps  in  a  cluster  such  as  that  shown  in  Fig.  53.  We 
have  no  reason  to  think  that  a  collision  between  two 
actual  bright  stars  was  the  origin  of  the  primaeval 
nebula  of  our  system.  But  when  we  reflect  that  the 
stars,  properly  so  called,  are  but  the  visible  members 
of  an  enormously  greater  host  of  objects,  then  the 
possibilities  of  occasional  collision  between  a  pair  of 
these  incomparably  more  abundant  dark  bodies  seems 
to  merit  our  close  attention.  We  are  not  by  any  means 
claiming  that  such  collisions  occur  frequently.  But  what 
we  do  say  is,  that  if,  as  we  believe,  these  bodies  are  to 
be  reckoned  in  many  millions  of  millions,  then  it  does 
sometimes  happen  that  two  of  them,  moving  about  in 
space,  will  approach  together  sufficiently  to  give  rise  to 
a  collision.  It  was  from  some  such  collision  that  we 
believe  the  nebula  took  its  rise  from  which  the  solar 
system  originated. 

We  have  the  best  reason  for  knowing  that  celestial 
collisions  do  sometimes  occur.  It  will  be  in  the 
recollection  of  the  readers  of  this  chapter  that  in 
February,  1901,  the  astronomical  world  was  startled 
by  the  announcement  of  the  outbreak  of  a  new  star 
in  Perseus.  A  photograph  of  that  part  of  the  heavens 
had  been  taken  a  few  days  before.  There  were  the 
ordinary  stars,  such  as  existed  from  time  immemorial, 
and  such  as  have  been  represented  on  the  numerous 
maps  in  which  the  stars  are  faithfully  set  down.  But, 
on  February  22nd,  Dr.  Anderson,  already  famous  by 


TEE    NEW  STAR    IN  PERSEUS.  357 

similar  discoveries,  noticed  that  the  constellation  of 
Perseus  contained  a  star  which  he  had  not  seen 
before.  Instantly  the  astronomical  world  was  apprised 
by  telegraph  that  a  new  star  had  appeared  in  Perseus, 
and  forthwith  most  diligent  attention  was  paid  to  its 
observation.  Photographs  then  obtained  show  the 
stars  that  had  been  seen  there  before,  with  the  addi- 
tion of  the  new  star  that  had  suddenly  come  into 
view.  For  a  few  nights  after  its  discovery  the  object 
increased  in  lustre,  until  it  attained  a  brightness  as 
great  as  that  of  Capella  or  Vega.  But  in  this  state 
it  did  not  long  remain.  This  brilliant  object  began 
to  wane.  Presently  it  could  not  be  classed  as  a  star 
of  the  first  magnitude,  nor  yet  of  the  second,  and  then 
it  ran  down  until  a  little  below  the  third,  and  even 
below  the  fourth.  In  the  subsequent  decline  of  the 
star  there  were  several  curious  oscillations.  On  one 
night  the  star  might  be  seen,  the  next  night  it  would 
be  hardly  discerned,  while  the  night  after  it  had  again 
risen  considerably.  But,  notwithstanding  such  tem- 
porary rallies,  the  brightness,  on "  the  whole,  declined, 
until  at  last  the  star  dwindled  to  the  dimensions  of  a 
small  point  of  light,  scarcely  distinguishable  with  the 
naked  eye.  The  decline  was  apparently  not  so  rapid 
as  the  increase,  but  nevertheless  from  the  first  moment 
of  its  appearance  to  the  last  was  not  longer  than  a 
few  weeks. 

This  new  star  in  Perseus  established,  in  one  sense, 
a  record.     For  the  star  was  brighter  than  any  new  star 
which  had   been  noticed   since   the    days   of  accurate 
astronomical   observations.      Not  indeed  for  three  cen- 
turies had  a  star  of  such  lustre  sprung  into  existence. 
But  a  temporary  star,  such  as  this  was,  has  been  by 
24 
r 


358  THE   EARTH'S   BEGINNING. 

no  means  an  infrequent  occurrence.  Many  such  have 
been  recorded.  Those  who  have  been  acquainted  with 
astronomical  matters  for  thirty  years  will  recollect  four 
or  five  such  stars.  In  each  of  them  the  general 
character  was  somewhat  the  same.  There  was  a  sudden 
outbreak,  and  then  a  gradual  decline.  The  questions 
have  sometimes  arisen  as  to  whether  the  outbreak  of 
such  an  object  is  really  the  temporary  exaltation  of 
a  star  which  was  previously  visible,  or  whether  it  ought 
not  to  be  regarded  as  the  creation  of  a  totally  new  star. 
In  some  cases  it  does  seem  possible  that  a  new  star  may 
have  been  partly,  at  all  events,  due  to  a  large  increase 
of  brightness  of  some  star  which  had  been  known  before. 
In  the  case  of  Nova  Persei,  however,  we  have  the  best 
authority  that  this  is  not  the  case.  Professor  Pickering, 
the  distinguished  astronomer  of  Harvard  College  Ob- 
servatory, happened  to  photograph  the  region  in  which 
Nova  Persei  appeared  a  few  days  before  the  outbreak 
took  place.  He  tells  us  that  there  is  not  the  least 
indication  on  his  photograph  of  the  presence  of  a  star 
in  that  region. 

The  spectrum  of  Nova  Persei,  in  an  instrument  of 
sufficient  power,  appeared  a  truly  magnificent  object. 
Like  other  stellar  spectra,  it  displayed  the  long  line 
of  light  marked  with  the  hues  of  the  rainbow,  but  it 
was  unlike  the  spectra  of  ordinary  stars  in  respect 
of  the  enormous  enhancements  of  the  brightness  at 
various  parts  of  this  spectrum.  For  instance,  at  one 
end  of  the  long  coloured  band  a  brilliant  ruby  line 
glowed  with  a  lustre  that  would  at  once  attract  atten- 
tion, and  demonstrated  that  the  object  under  view  must 
be  something  totally  different  from  ordinary  stars.  This 
superb  feature  is  one  of  the  lines  of  hydrogen.  The 


ITS   SPECTRUM. 


359 


Fig.  54.— SPECTRUM  OF  NOVA  PERSEI  (1901). 
{Photographed  with  the  40  in.  Yerkes  Telescope  by  Mr.  Ferdinand  Ellerman.) 


presence  of  that  line  showed  that  in  the  source  from 
which  the  light  came  there  must  have  been  a  remarkable 
outbreak  of  incandescent  hydrogen  gas.  At  various 
points  along  the  spectrum  there  were  other  beautiful 
bright  lines  which,  in  each  case,  must  have  been  due  to 
glowing  gas.  Here  we  have  the  evidence  of  the  spectrum 
telling  us  in  unmistakable  language  that  there  were 
features  in  this  star  wholly  unlike  the  features  found 
in  any  ordinary  star.  It  is  impossible  to  dissociate  these 


360  THE   EARTH'S    BEGINNING. 

facts  from  the  history  of  the  star.  Much  of  what  we 
have  said  with  regard  to  the  spectrum  of  Nova  Persei 
might  be  repeated  with  regard  to  the  spectrum  of  the 
other  temporary  stars  which,  from  time  to  time,  have 
burst  forth.  In  each  case  the  spectrum  characteristic 
of  an  ordinary  star  is  present,  but  superadded  to  it 
are  bright  lines  which  indicate  that  some  great  con- 
vulsion has  taken  place,  a  convulsion  by  which  vast 
volumes  of  gas  have  been  rendered  incandescent.  In 
Fig.  54  we  show  the  spectrum  of  Nova  Persei  on  five 
dates,  from  February  27th  to  March  28th,  1901.  These 
photographs  were  taken  by  Mr.  Ferdinand  Ellerman 
with  the  great  telescope  of  the  Yerkes  Observatory. 
They  show  in  the  clearest  manner  the  bright  lines 
indicating  the  incandescent  gases. 

We  have  pointed  out  the  high  probability  that 
among  the  millions  and  millions  of  bodies  in  the 
universe  it  may  now  and  then  happen  that  a  collision 
takes  place.  Have  we  not  also  explained  how  the 
heat  generated  in  virtue  of  such  a  collision  might  be 
sufficient,  and,  indeed,  much  more  than  sufficient,  to 
raise  the  masses  of  the  two  colliding  bodies  to  a  state  of 
vivid  incandescence?  A  collision  affords  the  simplest 
explanation  of  the  sudden  outbreak  of  the  star,  and 
also  accounts  for  the  remarkable  spectrum  which  the 
star  exhibits. 


CHAPTER    XIX. 

CONCLUDING    CHAPTER. 

Comprehensiveness  of  the  Nebular  Theory— Illustration— Huxley  and  the 
Origin  of  Species— Rudimentary  Organs— The  Apteryx— Its  Evanes- 
cent Wings— The  Skeleton — An  Historical  Explanation— Application 
of  the  Same  Method  to  the  Nebular  Theory— The  Internal  Heat  of 
the  Earth— The  Lady  Psyche. 

IT  is  not  difficult  to  show  that  the  nebular  theory 
occupies  a  unique  position  among  other  speculations  of 
the  human  intellect.  It  is  so  comprehensive  that 
almost  every  conceivable  topic  will  bear  some  relation 
to  it.  Perhaps  I  may  venture  to  give  a  rather  curious 
illustration  of  this  fact,  which  was  told  me  many  years 
ago  by  one  who  attended  a  course  of  lectures  by  an 
eminent  Professor  in  the  medical  faculty  at,  let  us  say, 
Vienna.  The  subject  of  the  course  was  the  no  doubt 
highly  important,  but  possibly  not  generally  interest- 
ing, subject  of  "  inflammation."  I  think  I  am  right  in 
saying  that  the  course  had  to  last  for  six  months, 
because  the  subject  was  to  be  treated  with  character- 
istic breadth  and  profundity.  At  all  events,  I  distinctly 
remember  that  the  learned  Professor  commenced  his 


362  THE   EARTH'S   BEGINNING. 

long  series  of  professional  discourses  with  an  account 
of  the  nebular  theory,  and  from  that  starting  point 
he  gradually  evolved  the  sequence  of  events  which 
ultimately  culminated  in — inflammation  ! 

It  may  be  remembered  that  in  the  year  1880,  Pro- 
fessor Huxley  delivered  at  the  Royal  Institution  a 
famous  lecture  which  he  termed  "  The  Coming  of  Age 
of  the  Origin  of  Species."  Among  the  many  remarkable 
and  forcible  illustrations  which  this  lecture  contained, 
I  recall  one  which  brought  before  the  audience,  in 
the  most  convincing  manner,  the  truth  of  the  great 
Darwinian  Theory  of  Evolution.  Huxley  pointed  out 
how  the  discoveries  in  Biology,  during  the  twenty-one 
years  which  immediately  succeeded  the  publication  of 
the  "  Origin  of  Species,"  had  been  so  numerous  and  so 
important,  and  had  a  bearing  so  remarkable  on  the 
great  evolutionary  theory,  that  even  if  the  Darwinian 
Theory  had  not  been  formed  to  explain  the  facts  of 
Nature,  as  they  were  known  .at  the  time  when  Darwin 
published  his  immortal  book,  the  same  theory  would 
have  had  to  be  formed,  were  it  only  to  explain 
the  additional  facts  whicK  had  come  to  light  since 
the  great  theory  itself  had  been  first  given  to  the 
world. 

I  believe  we  may  use  similar  language  with  regard 
to  the  nebular  theory  and  its  great  founders,  Kant, 
Laplace,  and  Herschel.  If  the  facts  which  were  known 
to  these  philosophers  led  them  to  adopt  in  one  form  or 
another  that  view  of  the  Origin  of  the  Universe  which 
the  nebular  theory  suggests,  how  stands  the  theory  now 
in  the  light  of  the  additional  facts  that  have  been  since 
disclosed  ?  If  we  merely  took  the  discoveries  which 
have  been  made  since  the  last  of  the  three  great 


THE    WINGLESS  BIRD.  363 

philosophers  passed  away,  it  might  well  be  maintained 
that  a  nebular  theory  would  be  demanded  to  account 
for  the  facts  brought  to  light,  in  the  interval. 

The  argument  on  which  the  nebular  theory  of  the 
solar  system  is  founded  has  other  parallels  with  that 
wonderful  doctrine  of  Natural  Selection  by  which 
Darwin  revealed  the  history  of  life  on  our  globe.  It 
not  unfrequently  happens  that  an  animal  has  in  its 
organisation  some  rudiments  of  a  structure  which  is 
obviously  of  no  use  to  the  animal  in  his  present  mode 
of  life,  and  would  be  unintelligible  if  we  supposed  the 
animal  to  have  been  created  as  he  is.  A  curious 
instance  of  a  rudimentary  structure  is  furnished  in 
the  apteryx,  the  famous  wingless  bird  which  still  lives 
in  New  Zealand. 

The  arrival  of  civilisation  in  New  Zealand  seems 
likely  to  -be  accompanied  with  fatal  results,  so  far  as 
the  unfortunate  apteryx  is  concerned.  Weasels  and 
other  fierce  enemies  have  been  introduced,  with  which 
this  quaint  bird  of  antiquity  is  unable  to  cope.  The 
apteryx  is  defenceless  against  such  foes.  Nature  had 
not  endowed  it  with  weapons  wherewith  to  fight,  for 
it  had,  apparently,  no  serious  adversaries  until  these 
importations  appeared  in  its  island  home.  Unlike  the 
ostrich,  the  apteryx  has  neither  strength  to  fight  his 
enemies,  nor  speed  to  run  away  from  them,  though,  like 
the  ostrich,  it  has  no  wings  for  flight;  indeed,  the 
apteryx  has  no  wings  at  all  As  its  name  signifies, 
the  apteryx  is  the  wingless  bird.  Living  specimens  are 
still  to  be  seen  in  the  Zoological  Gardens.  The  special 
point  to  notice  is  that,  though  he  has  no  wings  what- 
ever, still  there  are  small  rudimentary  wing-bones  which 
can  be  easily  seen.  You  need  not  be  afraid  to  put 


364  THE    EARTH'S    BEGINNING. 

your  hand  on  the  apteryx,  and  feel  the  puny  little 
remnants  of  wings  (Fig.  55). 

If,  having  seen  the  bird  in  the  Zoological  Gardens, 
you  go  to  the  Natural  History  Museum,  you  will  there 
find  a  skeleton  of  the  apteryx  (Fig.  56).  Look  near  the  ribs 
in  the  photograph,  and  there  you  will  see  those  poor  little 
wing-bones — wing-bones  where  there  never  was  a  wing. 
From  our  present  point  of  view  these  wings  are,  how- 
over,  more  interesting  and  instructive  than  the  most 
perfect  wings  of  an  eagle  or  a  carrier-pigeon.  Those 
wings  in  the  apteryx  may  be  incapable  of  flight,  but 
they  are  full  of  instruction  to  the  lover  of  Nature.  As  it 
is  certain  that  they  are  absolutely  of  no  use  whatever  to 
the  bird,  we  may  well  ask,  why  are  they  there  ?  They 
are  not  there  to  give  assistance  to  the  bird  in  his 
struggle  for  life ;  they  cannot  help  him  to  escape  from 
his  enemies  or  to  procure  his  food;  they  cannot  help 
him  to  tend  and  nurture  the  young  one  which  is 
hatched  from  the  egg;  they  can  help  him  in  no  way. 
The  explanation  of  those  ineffectual  wings  is  historical. 
Those  bones  are  present  in  the  apteryx  simply  because 
that  bird  has  come  down  by  a  long  line  of  descent 
from  birds  which  were  endowed  with  genuine  wings, 
with  wings  which  enabled  them  to  fly  like  rooks  or 
partridges. 

But  if  this  be  the  explanation,  how  has  it  come  to 
pass  that  the  wings  have  dwindled  to  useless  little  bones  ? 
We  cannot  of  course  feel  certain  of  the  reason,  but 
it  seems  possible  to  make  surmises.  In  early  times 
winged  birds  flew  over  the  sea  into  New  Zealand, 
and  found  it  a  country  of  abundance,  as  many  other 
immigrants  have  done  in  later  times.  It  may  have 
been  that  the  food  in  New  Zealand  was  so  plentiful  that 


A   LAND   FOR   BIRDS. 


365 


Fig.  55. — THE  APTERYX  :  A  WINGLESS  BIRD  OF  NEW  ZEALAND. 

the  wants  of  the  birds  could  be  readily  supplied,  with- 
out the  necessity  for  ranging  over  large  tracts.  It  may 
have  been  that  the  newly  arrived  birds  found  that  they 
had  few  or  no  enemies  in  New  Zealand,  from  which 
flight  would  be  necessary  as  a  means  of  escape.  It  may 
possibly  have  been  both  causes  together,  and  doubtless 
there  must  have  been  other  causes  as  well.  The  fact  is, 
however,  certain,  that  in  the  course  of  long  generations 


366 


THE   EARTH'S    BEGINNING. 


Fig.  56. — SKELETON  OF  THE  APTERYX,  SHOWING  RUDIMENTARY  WINGS. 

this  bird  gradually  lost  the  power  of  flight.  Natural 
selection  decrees  that  an  organ  which  has  ceased  to 
serve  a  useful  purpose  shall  deteriorate  in  the  course  of 
generations.  If  the  wings  had  become  needless  in  the 
search  for  food,  unnecessar}7  for  escape  from  enemies, 
and  useless  for  protection  of  its  young,  they  would  cer- 
tainly tend  towards  disappearance.  The  organism  finds 
it  uneconomical  to  maintain  the  nutrition  of  a  struc- 
ture which  discharges  no  useful  end.  The  wings,  in 


THE    WINGS    WERE   LOST.  367 

such  circumstances,  would  be  an  encumbrance  rather 
than  an  aid,  and  so  we  may  readily  conjecture  that,  in 
accordance  with  this  well-known  principle,  the  wings 
gradually  declined,  until  they  ceased  to  be  useful  organs, 
so  that  now  merely  a  few  rudimentary  bones  remain  to 
show  that  the  bird's  ancestors  had  once  been  as  other 


Fig.  57.— FORAMINIFER.  Fig.  58.— NAUTILUS. 

SPIRALS  IN  OTHER  DEPARTMENTS  or  NATURE. 

birds.  Whatever  may  have  been  the  cause,  it  seems 
certain  that  in  the  course  of  thousands  of  years,  or  it 
may  be  in  scores  of  thousands  of  years,  these  birds 
lost  the  power  of  flight ;  thus  they  gradually  ceased  to 
have  wings,  and  these  little  bones  are  all  that  now 
remain  to  render  it  almost  certain  that,  if  we  could 
learn  what  this  bird's  ancestry  has  been,  we  should  find 
that  it  was  descended  from  a  bird  which  had  useful 
wings  and  vigorous  flight.  Whenever  we  find  an  organ 
which  is  obviously  rudimentary,  or  of  no  use  to  its 
possessor  in  its  present  form,  Darwin  has  taught  us  to 
look  for  an  historical  explanation.  Let  us  see  if  we 
cannot  apply  this  principle  to  the  illustration  of  the 
nebular  theory. 

We    liken    the   internal   heat   of  the   earth  to  the 
rudimentary  wing  bones  of  the  apteryx.     In  each  case 


368  THE   EARTH'S   BEGINNING. 

we  find  a  survival  devoid  of  much  significance,  unless 
in  regard  to  its  historical  interpretation.  But  that 
historical  significance  can  hardly  be  over-estimated. 
Unimportant  as  the  wing-bones  may  be,  they  admit 
of  explanation  only  on  the  supposition  that  the 
apteryx  was  descended  from  a  winged  ancestor.  Un- 
important as  the  internal  heat,  still  lingering  in  our 
globe,  may  seem,  it  admits  of  explanation  only  on  the 
supposition  that  the  earth  has  had  the  origin  which 
the  nebular  theory  suggests. 

That  the  earth's  beginning  has  been  substantially 
in  accordance  with  the  great  Nebular  Theory  is,  I 
believe,  now  very  generally  admitted.  But  the  only 
authority  I  shall  cite  in  illustration  of  this  final  state- 
ment is  the  Lady  Psyche,  who  commences  her  ex- 
quisite address  to  her  "  patient  range  of  pupils  "  with 
the  words: — 

"This  world  was  once  a  fluid  haze  of  light, 
Till  toward  the  centre  set  the  starry  tides, 
And  eddied  into  suns,  that  wheeling,  cast 
The  planets ; " 


APPENDICES. 


I.-ON  THE  HEAT  GIVEN  OUT  IN  THE  CON- 
TRACTION OF  THE  NEBULA. 

§  1.    FUNDAMENTAL  THEOREMS  IN  THE  ATTRACTION  OF 
GRAVITATION. 

The  first  theorem  to  be  proved  is  as  follows  : — 
The  attraction  of  a  thin  homogeneous  spherical  shell  on  any 
point  in  its  interior  vanishes. 

Take  any  point  P  within  the  sphere.     Let  this  be  the  vertex 
of   a    cone    produced 

both   ways,   but  with  O 

a  very  small  vertical 
angle,  so  that  the 
small  areas  S  and  S', 
in  which  the  two 
parts  of  the  cone  cut 
the  sphere,  may  be 
regarded  as  planes. 
Draw  the  tangent 
planes  at  S  and  S'.  Q' 

Let  the  plane  of  the 
paper  pass  through  P 
and  be  perpendicular 
to  both  these  tangent  planes.  Let  O  P  O'  be  one  of  the  generators 
of  the  cone,  and  let  fall  P  Q  perpendicular  to  the  tangent  plane 
at  P,  and  O  Q'  perpendicular  to  the  tangent  plane  at  0'.  The 
volume  of  the  cone  with  the  vertex  at  P  and  the  base  S  is 


370  THE    EARTH'S   BEGINNING. 

J  P  Q  X  S,  and  the  other  part  of  the  cone  has  the  volume 
*  P  Q'  x  S'. 

As  the  vertical  angles  of  the  cones  are  small,  their  volumes 
will,  in  the  limit,  be  in  the  ratio  of  O  P3  to  O'  P3,  and  accordingly 
iPQ.S-hiPQ'.S'^PO'H-O'P3.  But  from  the  figure  PQ-H 
P  Q'  =  P  O  -J-  P  O',  and  hence  S  -^  O  P2  =  S'  -f-  O'  P2. 

As  the  shell  is  uniform,  the  masses  of  the  parts  cut  out  by 
the  cones  are  respectively  proportional  to  S  and  S'.  Hence  we  see 
that  the  attractions  of  S  and  S'  on  P  will  neutralise.  The  same 
must  be  true  for  every  such  cone  through  P,  and  accordingly 
the  total  attraction  of  the  shell  on  a  particle  inside  is  zero. 

The  second  fundamental  theorem  is  as  follows  : — 

A  thin  spherical  homogeneous  shell  produces  the  same 
attraction  at  an  external  point  as  if  its  entire  mass  were  con- 
centrated at  the  centre  of  the  sphere. 

This  is  another  famous  theorem  due  to  Newton.  He  gives  a 
beautiful  geometrical  proof  in  Section  XII.  of  the  first  book  of 
the  "Principia."  We  shall  here  take  it  for  granted,  and  we 
shall  consequently  assume  that — 

The  attraction  l>y  the  law  of  gravitation  of  a  homogeneous 
sphere  on  an  external  point  is  the  same  as  if  the  entire  mass  of 
the  sphere  were  concentrated  at  its  centre. 

§  2.    ON  THE  ENERGY  BETWEEN  Two  ATTRACTING  MASSES. 

Let  m  and  m'  be  two  attracting  bodies  supposed  to  be  small 
in  comparison  with  their  distance  x.  Let  the  force  between 
them  be  *  m  m'  -4-  #2  when  e  is  the  force  between  two  unit  masses 
at  unit  distance.  It  is  required  to  find  the  energy  necessary 
to  separate  them  to  infinity,  it  being  supposed  that  they  start 
from  an  initial  distance  a.  The  energy  required  is  obtained 
by  integrating  between  the  limits  infinity  and  a,  and  is  conse- 
quently fmm'-^-  a. 

§  3.    ON  THE  ENERGY  GIVEN  OUT  IN  THE  CONTRACTION  OF 

THE  NEBULA. 

We  assume  that  the  nebula  is  contracting  symmetrically, 
so  that  at  any  moment  it  is  a  homogeneous  sphere.  We  shall 
consider  the  shell  which  lies  between  the  two  spheres  of  radii, 
r  +  d  r  and  r  respectively. 

Let  M'  be  the  mass  of  the  nebula  contained  within  the  sphere 
of  radius  r,  and  let  d  M'  be  the  mass  of  the  shell  just  defined. 
Then  it  follows  from  S  1  that  the  condensation  of  the  shell  will 


APPENDICES.  371 

have  been  effected  by  the  attraction  of  the  mass  M'  solely. 
The  exterior  parts  of  the  nebula  can  have  had  no  effect,  for 
the  outer  part  has  always  been  in  symmetrical  spherical  shells 
exterior  to  d  M',  and  the  attraction  of  these  is  zero.  We  see 
from  §  2  that  the  contraction  of  d  M'  from  infinity,  until  it 
forms  a  shell  with  radius  r,  represents  a  quantity  of  energy, 

€  M'  d  W 

r 

for  it  is  obvious  that  the  energy  involved  in  the  contraction  of 
the  whole  shell  is  the  sum  of  the  energies  corresponding  to  its 
several  parts. 

If  M  be  the  total  mass  and  a  the  radius  of  the  nebula  always 
supposed  homogeneous 

M'  =  M  £, 

and  therefore  d  M'  =  3  M  —r  dr. 

a 

Hence  the  work  done  in  the  contraction  is 

i*y-*x£dr=*l 

Integrating,  therefore,  the  total  work  of  contraction  is 


5      a 

At  the  present  moment  a  mass  of  1  Ib.  at  the  surface  of  the 
sun  would  weigh  27  Ibs.  if  tested  by  a  spring  balance.    Hence 


With  this  substitution  Ave  find  the  expression  for  the  foot-pounds 
of  work  corresponding  to  the  contraction  of  the  nebula  from 
infinity  to  a  sphere  of  radius  a  to  be, 

f  .  27  a  M  =  16  a  M  very  nearly. 

Hence  we  have  the  following  fundamental  theorem  due  to 
Helmholtz,  which  is  the  basis  of  the  theory  of  sun  heat. 

If  the  sun  be  regarded  as  a  homogeneous  sphere  of  mass 
M  pounds  and  radius  a  feet,  then  the  foot-pounds  of  energy 
rendered  available  for  sun  heat  by  the  contraction  of  the  solar 
material  from  an  infinite  distance  is  16  a  M. 

§  4.    EVALUATION  OF  THE  SUN  HEAT  GIVEN  OUT  IN 

CONTRACTION. 

The  number  of  foot-pounds  of  work  given  out  in  the  con- 
traction from  infinity  is  16  a  M.  As  772  foot-pounds  are  equal  to 


372  THE    EARTH'S    BEGINNING. 

one  unit  of  heat,  i.e.  to  the  quantity  of  heat  necessary  to  raise 
1  Ib.  of  water  1°  Fahrenheit,  we  see  that  772  M  is  the  work 
required  to  raise  a  mass  of  water  equal  to  the  mass  of  the 
sun  through  1°  Fahrenheit.  Hence  the  number  of  globes  of 
water,  each  equal  to  the  sun  in  mass,  which  would  be  raised  1° 
Fahrenheit  by  the  total  heat  arising  from  the  contraction,  is 

16  a 
772' 

but  a,  the  radius  of  the  sun  in  feet,  is  2,280,000,000,  and  hence  we 
have  the  following  theorem  :  — 

The  energy  liberated  in  the  contraction  of  the  sun  from  infinity 
to  its  present  dimensions  would,  if  turned  into  heat,  suffice  to  raise 
47,000,000  globes  of  water,  each  having  the  same  mass  as  the  sun, 
through  1°  Fahr. 

It  is  found  by  experiment  that  1  Ib.  of  good  coal  may  develop 
14,000  units  of  heat,  and  is  therefore  equivalent  to  14,000  X  772 
foot-pounds  of  work.  A  mass  of  coal  equal  to  the  sun  would 
therefore  (granted  oxygen  enough)  be  equivalent  to  14,000  x  772 
x  M  foot-pounds  of  work.  But  we  have 

16  cTM  =  16  X  2,280,000,000 

14,000  X  772  X  M    "  14,000  X  772  3>4° 

Hence  we  see  that 

The  energy  liberated  in  the  contraction  of  the  sun  from  infinity 
to  its  present  dimensions,  is  as  great  as  could  be  produced  by  the 
combustion  of  3,400  globes  of  coal,  each  as  heavy  as  the  sun. 

We  may  speak  of  3,400  in  this  case  as  the  coal  equivalent. 

§  5.  ON  THE  FURTHER  CONTRACTION  OF  THE  SUN  AND  THE 
HEAT  THAT  MAY  THUS  BE  GIVEN  OUT. 

Let  us  suppose  the  sun  contracts  to  the  radius  r,  and  then,  as 
already  proved,  §  3,  the  energy  it  gives  out  is 


but  we  have 


whence  on  contraction  to  the  radius  r  the  total  energy  given  out 
from  the  commencement  is 

16  M 


APPENDICES.  373 

The  average  density  of  the  sun  at  present  is  1*4.  Let  us 
suppose  it  condenses  until  it  has  a  density  p. 

r3  -r-  a3  =  1'4  -f-  ?, 
whence  the  energy  becomes 

14aM.V7; 

but  the  coal  equivalent  of  16  a  M  has  been  found  in  §  4  to  be 
3,400,  and  hence  the  coal  equivalent  in  this  case  is 

3,000  V7 

If  we  take  P  to  be  the  density  of  platinum  (21*5),  we  get  a  coal 
equivalent  8,300.  This,  therefore,  seems  to  represent  a  major  limit 
to  the  quantity  of  heat  which  can  be  obtained  from  the  condensa- 
tion of  the  nebula  from  infinity  into  a  sun  of  the  utmost  density. 

*   §  6.    ON  THE  PRESENT  EMISSION  OF  SUN  HEAT. 

According  to  Scheiner,  "  Strahlung  und  Ternperatur  der  Sonne, 
Leipzig,  1899,"  the  value  of  the  solar  constant,  i.e.  the  number 
of  cubic  centimetres  of  water  which  would  be  raised  1°  Centigrade 
by  the  quantity  of  sun  heat  which,  if  there  were  no  atmospheric 
absorption,  would  fall  perpendicularly  on  a  square  centimetre,  at 
the  earth's  mean  distance  from  the  sun,  is  between  3'5  and  4'0. 
If  we  take  the  mean  value,  we  have  (translated  into  British  units), 
the  following  statement : — 

//  at  a  point  in  space,  distant  from  the  sun  by  the  earth's  mean 
distance,  one  square  foot  was  exposed  perpendicularly  to  the  solar 
rays,  then  the  sun  heat  that  would  fall  upon  it  in  one  minute  would 
raise  one  pound  of  water  14°  Fahr. 

This  shows  that  the  solar  energy  emitted  daily  amounts  to 
700,000,000,000  X  4  ir  a2  foot-pounds. 

§  7.    ON   THE  DAILY  CONTRACTION   OF  THE  SUN   NECESSARY 
TO  SUPPLY  THE  PRESENT  EXPENDITURE  OF  HEAT. 

We  have  seen  that  at  the  radius  r  the  energy  is 

16  Ma-2- 
Hence  for  a  change  d  r  it  is 

-  16  M  ^  dr. 

At  its  present  size,  accordingly,  the  energy  given  out  by  a 
shrinkage  d  r  is  16  M  d  r. 

25 


374  THE   EARTH'S    BEGINNING. 

One  cubic  foot  of  the  sun  averages  87  pounds,  so  that 
M  =  |  v  a3  X  87 

IGMdr  =  464  X  4*  a3  dr. 

We  have  to  equate  this  to  the  expression  in  the  last  article,  and 
we  get 

,         700,000,000,000 


This  is  the  shrinkage  of  the  sun's  radius  expressed  in  feet.     Hence 
the  daily  reduction  of  the  sun's  diameter  is  16  inches. 

One  coal  equivalent  possesses  energy  represented  by  M  x  1,400 
X  772.  Hence  we  can  calculate  that  one  coal  equivalent  would 
supply  the  solar  radiation  at  its  present  rate  for  about  2,800  years. 

II.—  THE    CONSERVATION    OF    MOMENT    OF 
MOMENTUM. 

We  give  -here  an  elementary  investigation  of  the  fundamental 
dynamical  principle  which  has  been  of  such  importance  through- 
out this  volume. 

§  8.      CASE   WHERE   THERE   ARE   NO   FORCES. 

Newton's  first  law  of  motion  tells  us  that  a  particle  in  motion 
if  unacted  upon  by  force,  will  move  continuously  in  a  straight 
line  without  change  of  velocity. 

Let  A0,  Fig.  60,  be  the  position  of  the  particle  at  any  moment. 
Let  A!  be  its  position  after  the  time  t  ;  A2  be  the  position  at 
the  time  2  t  ;  A3  be  the  position  at  the  time  3  1,  and  so  on. 

Then  the  first  law  of  motion  tells  us  that  the  distances 
A0  Aj,  A!  A2,  A2  A3,  A3  A4,  must  form  parts  of  the  same  straight 
line  and  must  be  all  equal. 

If  lines  O  A0,  OA1}  O  Aa,  etc.,  be  drawn  from  any  fixed  point 
O,  then  the  areas  of  the  triangles  OAoAj,  OAjA2,  OA2A3, 
O  A3  A4,  will  be  all  equal.  For  each  area  is  one  half  the  product 
of  the  base  of  the  triangle  into  the  perpendicular  O  T  from  O 
on  A0  AU  and,  as  the  bases  of  all  the  triangles  are  equal,  it  follows, 
that  their  areas  are  equal. 

Thus  we  learn  that  a  particle  moving  without  the  action  of 
force  will  describe  around  any  fixed  point  O  equal  areas  in 
equal  times. 

The  product  of  the  mass  of  the  particle  and  its  velocity  is 
termed  the  momentum.  If  the  momentum  be  multiplied  by 


APPENDICES. 


375 


OT  the  product  is 
termed  the  moment 
of  momentum  around 
O.  We  have  in  this 
case  the  simplest  ex- 
ample of  the  import- 
ant principle  known  as 
the  conservation  of  mo- 
ment of  momentum. 

The  moment  of 
momentum  of  a  sys- 
tem of  particles  mov- 
ing in  a  plane  is 
defined  to  be  the 
excess  of  the  sum  of 
the  moments  of  mo- 
mentum of  those  par- 
ticles  which  tend 
round  O  in  one  direc- 
tion, over  the  sum  of  the  moments  of  momentum  of  those 
particles  which  tend  round  O  in  the  opposite  direction. 

If  we  deem  those  moments  in  one  direction  round  O  as 
positive,  and  those  in  the  other  direction  as  negative,  theti  we 
may  say  that  the  moment  of  momentum  of  a  system  of  particles 
moving  in  a  plane  is  the  algebraical  sum  of  the  several  moments 
of  momentum  of  each  of  the  particles. 


Fig.  60. — FIRST  LAW  OF  MOTION  EXEMPLIFIES 
CONSTANT  MOMENT  OF  MOMENTUM. 


9.    A  GEOMETRICAL  PROPOSITION. 
theorem    in    elementary    geometry 


will    be 


The    following 
required  : — 

Let  A  B  and  A  C  be  adjacent  sides  of  a  parallelogram,  Fig.  61 , 
of  which  A  D  is  the  diagonal,  and  let  O  be  any  point  in  its  plane. 
Then  the  area  O  A  C  is  the  difference  of  the  areas  O  A  D 
and  O  A  B. 

Draw  I)  Q  and  C  P  parallel  to  O  A.  Then  O  A  D  =  O  A  Q, 
whence  OAD  -  OAB  =  OBQ  =  OAP  =  OAC. 

§  10.    RELATION    BETWEEN    THE    CHANGE    OF    MOMENT    OF 
MOMENTUM  AND  THE  FORCE  ACTING  ON  THE  PARTICLE. 

Let  AI  and  A2,  Fig.  62,  be  two  adjacent  points  on  the  path  of  the 
particle,  and  let  AI  Q  and  A2  Pt  be  the  tangents  at  those  points. 


376 


THE    EARTH'S    BEGINNING. 


Fig.  61. — A  USEFUL  GEOMETRICAL  PROPOSITION. 

Let  S  Q  represent  the  velocity  of  the  particle  at  At,  and  S  R  the 
velocity  of  the  particle  at  A2.  Then  Q  R  represents  both  in 
magnitude  and  direction  the  change  in  velocity  due  to  the  force  F 
which  we  suppose  constant  both  in  magnitude  and  direction 
while  the  particle  moves  from  Ax  to  A2  in  the  small  time  t ;  we 
have  also  Q  R  =  F  t  -f-  m. 

Complete  the  parallelogram  SQRU,  and  let  fall  OP15  OP2, 


Fig.   62.— ACCELERATION  OF  MOMENT  OF  MOMENTUM  EQUALS  MOMENT 
OF  FORCE. 


APPENDICES.  377 

O  T  perpendiculars  from  O  on  S  Q,  S  R,  S  U  respectively.  Since 
SQ  is  the  velocity  of  the  particle  when  at  AI  the  moment  of 
momentum  is  m  O  PI  x  S  Q ;  when  the  particle  is  at  A2  the 
moment  of  momentum  is  m  O  Pa  x  S  R.  Whence  the  difference 
of  the  moments  of  momentum  at  Aa  and  A2  is  m  (O  P2  X  S  R  - 
O  P!  x  S  Q)  =  2  m  (O  S  R  -  O  S  Q)  =  2  m  O  S  U  =  ra  O  T  x 
S  U  =  m  O  T .  Q  R  =  F  £  x  O  T.  But  in  the  limit  S  coincides 
with  Ai  and  A2,  and  we  see  that  the  gain  in  moment  of  momentum 
is  t  times  the  moment  of  the  force  around  O.  Hence  we  deduce 
the  following  fundamental  theorem,  in  which,  by  the  expression 
acceleration  of  moment  of  momentum,  we  mean  the  rate  at  which 
the  moment  of  momentum  increases  : — 

If  a  particle  under  the  action  of  force  describes  a  plane  orbit, 
then  the  acceleration  of  the  moment  of  momentum  around  any 
point  in  the  plane  is  equal  to  the  moment  of  the  force  around 
the  point. 

If  the  force  is  constantly  directed  to  a  fixed  point,  then  the 
moment  of  the  force  about  this  point  is  always  zero.  Hence 
the  acceleration  of  the  moment  of  momentum  around  this  point 
is  zero,  and  the  moment  of  momentum  is  constant.  Thus  we 
have  Kepler's  law  of  the  description  of  equal  areas  in  equal 
times,  and  we  learn  that  the  velocity  is  inversely  proportional 
to  the  perpendicular  on  the  tangent. 

§11.  IF  Two  OR  MORE  FORCES  ACT  ON  A  POINT,  THEN  THE 
ACCELERATION  OF  THE  MOMENT  OF  MOMENTUM,  DUE  TO 
THE  RESULTANT  OF  THESE  FORCES,  is  EQUAL  TO  THE 
ALGEBRAIC  SUM  OF  THE  MOMENTS  OF  MOMENTUM  DUE  TO 
THE  ACTION  OF  THE  SEVERAL  COMPONENTS. 

Let  A  D,  Fig.  61,  be  a  force,  and  A  C  and  A  B  its  two  components. 
Then,  since  OAD  =  OAB  +  OAC,  we  see  that  the  moment  of 
AD  around  O  is  equal  to  the  sum  of  the  moments  of  its  com- 
ponents. Hence  we  easily  infer  that  if  a  force  be  resolved 
into  several  components  the  moment  of  that  force  around  a 
point  is  equal  to  the  algebraical  sum  of  the  moments  of  its 
several  components. 

The  acceleration  of  the  moment  of  momentum  around  O, 
due  to  the  resultant  of  a  number  of  forces,  is  equal  to  the 
moment  of  that  resultant  around  O.  But,  as  we  have  just 
shown,  this  is  equal  to  the  sum  of  the  moments  of  the  separate 
forces,  and  hence  the  theorem  is  proved. 


378  THE    EARTH'S    BEGINNING. 

§12.  IF  ANY  NUMBER  OF  PARTICLES  BE  MOVING  IN  A  PLANE, 
AND  IF  THEY  ARE  NOT  SUBJECTED  TO  ANY  FORCES  SAVE 
THOSE  WHICH  ARISE  FROM  THEIR  MUTUAL  ACTIONS,  THEN 
THE  ALGEBRAIC  SUM  OF  THEIR  MOMENTS  OF  MOMENTUM 
ROUND  ANY  POINT  IS  CONSTANT. 

This  important  theorem  is  deduced  from  the  fact  stated  in 
the  third  law  of  motion,  that  action  and  reaction  are  equal  and 
opposite.  Let  us  take  any  two  particles;  then,  the  acceleration 
of  the  moment  of  momentum  of  one  of  them,  A,  by  the  action 
of  the  other,  B,  will  be  the  moment  of  the  force  between 
them.  The  acceleration  of  the  moment  of  momentum  of  B  by 
the  action  of  A  will  be  the  same  moment,  but  with  an  opposite 
sign.  Hence  the  total  acceleration  of  the  moment  of  momentum 
of  the  system  by  the  mutual  action  of  A  and  B  is  zero.  In 
like  manner  we  dispose  of  every  other  pair  of  actions,  and  thus, 
as  the  total  acceleration  of  the  moment  of  momentum  is  zero,  it 
follows  that  the  moment  of  momentum  of  the  system  itself  must 
be  constant. 

This  fundamental  principle  is  also  known  as  the  doctrine  of 
the  conservation  of  areas.  It  may  be  stated  in  the  following 
manner  : — 

If  a  system  of  particles  are  moving  in  a  plane  under  the 
influence  of  their  mutual  actions  only,  the  algebraic  sum  of  the  areas 
swept  out  around  a  point,  each  multiplied  by  the  mass  of  the 
particle,  is  directly  proportional  to  the  time. 

§  13.    IF  A  PARTICLE  OF  MASS  ra,  is  MOVING  IN  SPACE  UNDER 

THE   ACTION    OF     ANY    FORCE     F,    THEN    THE   PROJECTION   OF 

THAT  PARTICLE  ON  ANY  FIXED  PLANE  WILL  MOVE  AS  IF 
IT  WERE  A  PARTICLE  OF  MASS  m  ACTED  UPON  BY  THAT 
COMPONENT  OF  F  WHICH  is  PARALLEL  TO  THE  PLANE. 

This  is  evident  from  the  consideration  that  the  acceleration 
of  the  particle  parallel  to  the  plane  must  be  proportional  to 
this  component  of  F. 

Let  us  now  suppose  a  system  of  particles  moving  in  space 
under  their  mutual  actions.  The  projections  of  these  particles 
on  a  plane  will  move  as  if  they  were  the  particles  themselves 
subjected  to  the  action  of  forces  which  are  the  projections  of 
the  actual  forces  on  the  same  plane,  and  as  the  reactions  between 
any  two  particles  are  equal  and  opposite,  the  projections  of 


APPENDICES.  379 

tnoce  reactions  on  the  plane  are  equal  and  opposite.  Hence  the 
proof  already  given  of  the  constancy  of  the  moments  of  momentum 
of  a  plane  system,  will  apply  equally  to  prove  the  constancy  of  the 
moments  of  momentum  of  the  projections  of  the  particles  on  the 
plane.  Hence  we  have  the  following  important  theorem  : — 

Let  a  system  of  particles  be  moving  in  space  under  the  action 
of  forces  internal  to  the  system  only.  Let  any  plane  be  taken,  and 
any  point  in  that  plane,  and  let  the  momentum  of  each  particle  be 
projected  into  the  plane,  then  the  algebraic  sum  of  the  moments  of 
these  projections  around  the  point  is  constant. 

§  14.    ON  THE  PRINCIPAL  PLANE  OF  A  SYSTEM. 

Let  us  suppose  a  system  of  particles  moving  under  the 
influence  of  their  mutual  actions.  Let  O  be  any  point,  and 
draw  any  plane  L  through  O.  Then  the  moment  of  momentum 
of  the  system  around  the  point  O  and  projected  into  the  plane 
L  is  constant.  Let  us  call  it  S.  If  another  plane,  L',  had 
been  drawn  through  O,  the  similar  moment  with  regard  to  L' 
is  S'.  Thus  for  each  plane  through  O  there  will  be  a  corre- 
sponding value  of  S.  We  have  now  to  show  that  one  plane 
can  be  drawn  through  O,  such  that  the  value  of  S  is  greater 
than  it  is  for  any  other  plane.  This  is  the  principal  plane  of 
the  system. 

If  v  be  the  velocity  of  a  particle,  then  in  a  small  time  t  it 
moves  over  the  distance  v  t.  If  p  be  the  perpendicular  from  O 
on  the  tangent  to  the  motion,  then  the  area  of  the  triangle 
swept  round  O  in  the  time  t  is  %pvt,  and  we  see  that  the 
momentum  is  proportional  to  the  mass  of  the  particle  multiplied 
into  the  area  swept  over  in  the  time  t.  The  quantity  S  will,  there- 
fore, be  proportional  to  the  sum  of  the  projections  of  the  areas  in 
L,  swept  over  in  the  time  t,  each  increased  in  the  proportion  of 
the  mass  of  the  particle.  It  is  easily  seen  that  the  projection  of 
an  area  in  one  plane  on  another  is  obtained  by  multiplying  the 
original  area  by  the  cosine  of  the  angle  between  the  two  planes. 
For  if  the  area  be  divided  into  thin  strips  by  lines  parallel  to  the 
line  of  intersection  of  the  planes,  then  in  the  projection  of  these 
strips  the  lengths  are  unchanged,  while  the  breadths  are  altered 
by  being  multiplied  by  the  cosine  of  the  angle  between  the 
two  planes.  If,  therefore,  we  mark  off  on  the  normal  to  a  plane  L 
a  length  h  proportional  to  any  area  in  that  plane,  then  the 


380  THE   EARTH'S    BEGINNING. 

projection  of  this  area  on 
any  other  plane  L'  may 
be  measured  by  the  pro- 
jection of  h  on  the  nor- 
mal to  L'. 

To  determine  the  mo- 
ment of  momentum  re- 
solved in  any  plane -we 
therefore  proceed  as  fol- 
lows :  Draw  a  plane 
through  O,  and  the  tan- 
gent to  the  path  of  one 
of  the  particles,  and 
mark  off  on  the  normal 
drawn  through  O  to  this 
plane  a  length  I  propor-  FiS'  63.— MOMENT  OF  MOMENTUM  UNALTERED 
tional  to  the  moment  of  BY  CoLLISION- 

momentum.    .Repeat  the 

same  process  for  each  of  the  other  particles  with  lengths  I',  l"t 
etc.,  on  their  several  normals.  Suppose  that  /,  /',  I"  represent 
forces  acting  at  O,  and  determine  their  resultant  R.  Then  R, 
resolved  along  any  other  direction,  will  give  the  component  of 
moment  of  momentum  in  the  plane  to  which  that  direction  is 
normal.  In  any  plane  which  passes  through  R  the  component 
of  moment  of  momentum  is  zero.  The  plane  perpendicular 
to  R  contains  the  maximum  projection  of  moment  of  momen- 
tum. This  is  the  principal  plane  of  the  system  which  we  have 
seen  to  be  of  such  importance  in  connection  with  the  nebular 
theory. 

§  15.    COLLISIONS. 

The  conservation  of  moment  of  momentum  remains  true  in 
a  system,  even  though  there  may  have  been  actual  collisions 
between  the  several  parts.  This  is  included  in  the  proof  already 
given,  for  collisions  are  among  the  mutual  actions  referred  to.  It 
may,  however,  be  instructive  to  give  a  direct  proof  of  a  particular 
case. 

Let  two  particles  collide  when  meeting  in  the  directions  A  P 
and  BP(Fig.  63)  respectively.  Whether  the  particles  be  elastic 
or  inelastic  is  quite  immaterial,  for  in  both  cases  the  action  and 
reaction  must  be  equal  and  opposite,  and  take  place  along  some 
line  P  Q.  The  action  on  the  particle  moving  along  A  P  will  give 


APPENDICES.  381 

to  it  an  acceleration  of  moment  of  momentum  which  is  equal  to 
the  moment  of  the  action  around  O.  The  acceleration  of  the 
moment  of  momentum  coming  along  B  P  will  be  equal  and  oppo- 
site. Thus  the  total  acceleration  of  the  moment  of  momentum 
is  zero.  Hence  the  collision  has  no  effect  on  the  total  moment 
of  momentum. 

§  16.    FRICTION  AND  TIDES. 

We  have  shown  that  such  actions  as  collisions  cannot  affect  the 
moment  of  momentum  of  the  system,  neither  can  it  be  affected 
by  friction  of  one  body  on  another.  Here,  as  in  the  former  case, 
the  actions  and  reactions  are  equal  and  opposite,  and  consequently 
the  accelerations  of  moment  of  momentum  are  zero.  Nor  is  it 
possible  for  any  tidal  action  to  affect  the  total  moment  of 
momentum  of  the  system.  Every  such  action  must  be  composed 
of  the  effects  of  one  particle  in  the  system  on  another,  and 
as  this  must  invariably  produce  an  equal  and  opposite  reaction 
the  total  moment  of  moment  urn  is  unaltered. 


INDEX. 


Acceleration  of  moment  of  momen- 
tum, 377 

Aldebaran,  27,  28 

Anderson,  Dr.,  356 

Andromeda,  Great  Nebula  in,  43,  204 

Antinous,  Cluster  of  stars  in,  353 

Apteryx,  Rudimentary  wing-bones 
of,  364,  366 

,  Skeleton  of,  364,  36b 

,  The,  363,  365 

Arcturus,  Spectrum  of,  85 

Argon,  265 

Argus  and  surrounding  stars3  103 

Ariel,  338 

Boring,  The  great,  123 
Brooks'  comet,  89 
Bunsen  burner,  The,  283 
Butterfly  and  the  oak-tree,  The,  15 

Calcium,  274 

Capella,  Spectrum  of,  61-64 

Carbon,  280 

Ceres,  311 

Change  of  moment  of  momentum,  375 

Cluster,  Nebulous  region  round  a,  33 

Clusters  of  stars,  53-60,  203 

of  17th  magnitude,  353 

Coal-unit,  110 
Collisions,  219,  380 

,  Cause  of  formation  of  nebulae, 

356 

Comets,  37 
Comet,  Brooks',  89 

of  1882,  119 

,  Spectrum  of,  290 

Common,  Dr.  A.  A.,  44 
Concord,  The  first,  294-307 

,  The  second,  308-323 

,  The  third,  324-336 


Conglomerates,  159 

Conservation  of  moment  of  momen- 
tum, 374 

Corona  of  the  sun,  117 

Crab  nebula,  The,  19,  44 

Crossley  Eeflector,  The,  45,  46,  48, 
49,  50,  67,  199 

Cyguus,  Nebula  in,  329 

Dark  bodies  in  universe,  355 
Darwin,  Professor  G.  H. ,  153,  254, 332 
Darwinian  theory,  10,  268,  362 
Dewar,  Professor,  144,  272 
Diurnal  motion,  The,  21 
Dumb-bell  nebula,  43,  44, 45,  46,  50, 

74,  195 
Dust  from  Krakatoa,  185 

Earth,  Heat  in  interior  of,  134,  367 

— , ,  Cause  of,  153 

— ,  History  of,  122-157,  251 

,  Rigidity  of,  162 

Earth-moon  system,  253,  332 
Earthquakes,  158-190 

in  England,  175 

,  Routes  of,  171 

Emission  of  sun  heat,  373 

Energy    between     two     attracting 

masses,  370 
given    out    in    contraction    of 

nebula,  370 

of  a  system,  216,  235 

Equivalent  of  heat,  88 

Eros,  312 

Evaluation  of  sun  heat  given  out  in 

contraction,  371 
Everett,  Professor,  149 

Fire-mist,  The,  268 
"Flash"  spectrum,  70 


INDEX. 


Foot-pound,  91 
Foraminifera,  367 
Friction  and  tides,  381 

Gas  in  rarefaction,  118 

H  and  K  lines,  70,  276 
Heat,  Cause  of,  153 

— ,  Equivalent  of,  88 
given    out    in    contraction   of 

nebula,  369 

iii  interior  of  the  earth,  134,  367 

— ,  Unit  of,  80,  89 
Helium,  277 
Helmholtz,  86,  96,  100 
Hercules,  Star- cluster  in,  52,  53,  56, 

57,59 

Herschel,  Sir  William,  4, 11, 72, 73,  74 
Huggins,  Sir  W.,  60,  61,  63,  65 
Huxley,  Professor,  and  Darwinian 

theory,  362 

Huyssen,  Captain,  127 
Hydrogen     in    spectrum    of    Nova 

Persei,  358 

"Inflammation"        and       nebular 
theory,  361 

Joule's  equivalent  of  heat,  88 
Jupiter,  23,  25,  26,  29,  208,  237,  310, 
327 

K  and  H  lines,  70,  276 

Kant,  Immauuel,  4,  5,  72,  73,  74,  327 

Keeler,  Professor,  45-48,  67,  73,  199, 

200,  202,  245 
Kelvin,  Lord,  153,  162 
Krakatoa,  176-189 

Langley,  Professor,  78 
Laplace,  4,  72,  73,  74,  206 
Lassell,  Mr.,  338,  340 
Lick  Observatory,  41,  43,  44,  45 
Lockyer,  Sir  Norman,  277 
Lyra,  Ring  nebula  in,  249 

Mars,  25,  26,  27,  28,  29,  311,  335 

. ,  Satellites  of,  335 

Mecanique  Celeste,  5,  8 
Mercury,  23,  25,  26,  29,  208 


Meteors,  37 

Milky  Way,  205,  206,  214,  220 

Milne,  Professor,  165 

Moment  of  momentum,  222,  226,  240, 

352 

,  Acceleration  of,  377 

,  Change  of,  375 

,  Conservation  of,  374 

Momentum,   Moment  of,   222,  226, 

240,  352 
Monoceros,  Nebulous  region  round 

a  cluster  in,  33 
Moon,  Origin  of,  254 
,  Surface  of,  255 

Nautilus,  The,  367 

Nebula,  Contraction  of,  Heat  given 

out  in,  369 

, ,  Energy  given  out  in,  370 

in  Orion,  The  great,  40,  41,  42, 

44,  46,  50,  74,  195,  242 

— , ,  Spectrum  of,  63,64,65 

— — ,  The  great  spiral,  192,  193 
Nebula?,  40,  41,  43,  45,  47,  50,  57,  58, 

66,  67,  71, 73, 105, 120,  157, 191-206, 

242,  247,  249,  256,  257,  258,  259, 

296,  329,  345,  348-360 

,  Development  of,  242 

,  Discovery  of,  il 

,  Number  of,  67,  200 

Nebular  anecdote,  362 

Theory,  The,  V3,  72,  74,  157, 

205,  266,  292,  307,  323,  328,  331, 

337-347,  362,  368 
Nebulosity,      Faint      diffused,     in 

Perseus,  17 
Neptune,  37 

,  Satellites  of,  339,  340 

Newcomb,  Professor,  238 
Norway,  Conglomerates  in,  159 
Nova  Persei,  358 
,  Spectrum    of,   358,   359, 

360 

Oak-tree  and  the  butterfly,  The,  15 

Oberon,  338 

Orbits  of  the  planets,  208 

Orion,  22 


384 


INDEX. 


Orion,  Great  nebula  in,  40,  41,  42,  44, 

46,  50,  74,  195,  242 
,  Spectrum  of,  63,  64,  65 

Pegasus,  Nebula  in,  47,  345 
Perseus,  A  faint  diffused  nebulosity 

in,  17 

,  New  star  in,  356 

Photosphere,  The,  69 
Pickering,  Professor,  358 
"Plane,  Principal,"  The,  225,  352, 379 
Planetary  system,  The,  37,  208 
Planets,  22,  26,  28 

,  Movement  of,  35,  311 

,  Orbits  of,  208,  298 

— ,  Eotation  of,  on  their  axes,  325 
Platinum,  263 
Pleiades,  22 

,  Nebulae  in,  71 

Potassium,  272 

1  'Principal Plane,"  The,  225,  352, 379 

Probabilities,  Theory  of,  305 

Radiation  of  sun's  heat,  82 
Ramsay,  Professor,  278 
Ray  nebulae,  201,  211 
Rigidity  of  the  earth,  162 
Ring  nebula  in  Lyra,  The,  249 
Roberts,  Dr.  Isaac,  198 
Rosse,  Lord,  57,  196-201 
Rowland,  Prof.  Henry,  273 

Sagittarius,  Nebula  in,  105 

Satellites,  37,  209 

Saturn,  25,  26,  29,  220,  233 

,  Dweller  in,  and  the  Sirian,  14 

,  Ring  of,  210,  220,  231,  232,  233, 

234 

Scheiner,  Professor,  202,  204 
Seismometer,  The,  165 
Sirian,    The,    and   the    dweller    in 

Saturn,  14 
Sirius,  215,  216 

Smiths,  The  parable  of  the,  303 
Solar  system,  36,  207 

,  H  nergy  of,  350 

,  Evolution  of,  20,  246-260,  349 

,  Origin  of,  351 


Solar  system,  Scale  of,  29,  30,  31 
Spectra,  Continuous,  68,  203 

,  Discontinuous,  68 

Spectroscope,  The,  60,  271 
Spiral  form  in  Nature,  256,  257 
Spiral  nebula,  The  great,  192, 193, 247 
Spiral    nebulae,    191-206,    211,   212, 

213,  220,  243,  247,  256,  257,  258, 

259,  296,  345 
Star-clusters,  53-60 
Star,  Spectrum  of,  64 
Stars  distinguished  from  planets,  28, 

29 

Stoney,  Dr.  G.  Johnstone,  279 
Sun  compared  with  the  planets,  26,  29 

,  Corona  of,  117 

,  Contraction  of,  99,  373 

,  Density  of,  102,  115 

,  Heat  of,  75-94, 95-111,  371,  372, 

373 

,  History  of,  112-121,  251 

,  Nebulous  part  of,  121 

,  Spectrum  of,  61, 62, 69, 70, 85, 273 

,  Surface  of,  278 

,  Velocity  of,  354 

,  Weight  of,  101 

Sun  heat,  given  out  in  contraction, 

Evaluation  of,  371,  372 

,  Present  emission  of,  373 

Sunsets,  The  Krakatoa,  189 
Sysi$me  du  Monde,  8 

Thermometer    for  testing    the  heat 

of  the  earth's  interior,  129 
Thomson,  Prof.  J.  J.,  316 
Tides  and  friction,  381 
Titania,  338 

Umbriel,  338 
Unit  of  heat,  80,  89 
Uranus,  37,  238,  239 
,  Satellites  of,  238,  338 

Venus,  23,  25,  26,  29,  208,  325 
Volcanoes,  158-190 
Voltaire,  Fable  of ,  14 

Waves  caused    by  Krakatoa  earth- 
quake, 179,  182,  183 


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ICLS 

fN) 

LD 

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USE 

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LD 

R63 

1    Viw  *.*  n 

ill/ 

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Mfl     flMTERLIBRARY  LC 

IAN 

FEB  1  S  ]97° 

^G  2  0  197ft 

SEC.  Clg.      SEP  2  0  76 

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DEC* 

jOT)     DEC     1  198 

3 

MA' 

4 

1-10 

T  -not  A    «n«»  «  >«a                                 General  Library 
/  TOftofiT  Pn  vl"?ft?A  ^9                     University  of  California 
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^8 


